An Essay Concerning Human Understanding – John Locke

A Systematic Inquiry into the Origin, Certainty, and Extent of Human Knowledge

The Epistle to the Reader

We begin before the Essay proper, with Locke’s own preface — a letter to the reader explaining how this vast work came to be written. Locke tells us directly: this is not a work of systematic philosophy composed from a plan, but rather the result of “five or six friends meeting at my chamber, and discoursing on a subject very remote from this.” Finding themselves “quickly at a stand, by the difficulties that rose on every side,” Locke concluded that “we must examine our own abilities, and see what objects our understandings were, or were not, fitted to deal with.” That initial conversation, about morality and revealed religion, forced the question that would consume the next two decades of his intellectual life.

What follows is Locke’s account of how the Essay grew. He describes writing “incoherent parcels” and later connecting them into a coherent whole, publishing the first edition in 1689 (dated 1690) after returning from exile in Holland. He is defensive about his style — “I am not ignorant how little I herein consult my own reputation” — and admits that he has not “the learning to be a critic.” But he insists that his purpose is clarity rather than ornament: “I write only for the information of those who, being taught by their own experience, will not think it unreasonable to examine the grounds of their own opinions.” He warns the reader that the book is long, but argues that “the subject is so close and entangled, that it cannot be dispatched in fewer words.” The Epistle closes with a frank acknowledgment that he expects no universal acceptance: “I shall always have the satisfaction to have aimed sincerely at truth and usefulness.”

This opening matters because Locke establishes from the start that his project is not speculative system-building but an empirical investigation into the limits of human understanding itself. He admits his own fallibility and invites correction, but he does not apologize for the work’s scope. The Essay is presented as a necessary clearing of the ground before any serious inquiry into knowledge can proceed.

Book I — Of Innate Notions: Chapter I — Introduction

Locke opens the Essay proper with one of the most famous passages in modern philosophy. The reader is told that “it is a great deal of satisfaction to him who, being but a novice in these speculations, and having a full view of all that is to be found in them, shall be able to determine how far his understanding can reach, and when it becomes blind.” This is the work’s governing project — to chart the boundaries of human knowledge, not to extend it beyond its natural limits. Locke describes the understanding as “the most elevated faculty of the soul” and insists that studying it is “the first and most necessary” inquiry, because without knowing what our minds can and cannot do, we waste effort chasing impossibilities or, worse, fall into skepticism when we fail.

Locke then introduces the central question of the entire Essay: “Whence has the mind all the materials of reason and knowledge?” His answer is stated plainly and without qualification: “To this I answer, in one word, from experience.” All our ideas — all the content of our thinking — come either from sensation (perception of external objects) or reflection (perception of the internal operations of our own minds). Sensation provides ideas like yellow, white, heat, cold, soft, hard, bitter, sweet; reflection provides ideas like perception, thinking, doubting, believing, reasoning, knowing, willing. The mind, before any experience, is a tabula rasa — a blank slate, “white paper, void of all characters, without any ideas.” Everything written on that paper comes from experience alone.

Locke’s introduction also mounts an explicit attack on the doctrine of innate ideas — the belief that certain principles (logical axioms, moral rules, the idea of God) are imprinted on the human mind at birth. He calls this hypothesis “a supposition” that “will be of great use to lazy men, who will thereby avoid the pains of search” and “be an excuse for ignorance.” The rest of Book I will be devoted to demolishing this doctrine. For now, Locke merely announces his commitment to experience as the sole source of knowledge, and his intention to trace “the ways whereby we come to attain those notions of things we have.”

This introduction is the philosophical foundation of everything that follows. Locke is not arguing that knowledge is impossible or that reason is powerless; he is arguing that knowledge must be grounded in what we actually observe and experience. The Essay is therefore a work of empiricism — the claim that all content of the mind derives from sensory experience and introspection, and that any claim to knowledge must be traced back to those origins.

Book I — Chapter 2: No Innate Principles in the Mind

Locke opens his attack on innate ideas by addressing what he considers the most plausible candidates: speculative principles — universal truths of logic and metaphysics that are supposedly stamped on every human mind from birth. The most famous example is the principle of identity: “Whatsoever is, is.” Another is the principle of non-contradiction: “It is impossible for the same thing to be and not to be.” If these are innate, Locke argues, they must be universally known and assented to by all human beings, including children and “idiots.” But clearly, children and the mentally disabled do not possess such abstract notions. “Children and idiots have not the least apprehension or thought of them,” Locke writes. “And the want of that is enough to destroy that universal consent which must be the necessary concomitant of all innate truths.”

Locke then dismantles the claim that these principles are “imprinted” on the mind but not yet consciously noticed. This, he says, is nonsense: “No proposition can be said to be in the mind which it never yet knew, which it was never yet conscious of.” To say that a truth is in the mind but not known is like saying a body has a shape it never displays. He drives the point home: “If it be in the mind, and yet not known, that is all one as to say it is in the mind and yet not in the mind.” The defenders of innate ideas, Locke argues, have confused the mind’s capacity to know certain truths (when it acquires the relevant ideas) with the presence of those truths themselves from birth. A child does not know that “it is impossible for the same thing to be and not to be” until she has acquired the ideas of identity and contradiction through experience. The principle becomes known, but it is not born known.

Locke also addresses the argument that innate principles become known “upon the first use of reason.” This, he says, is equally false. If reason revealed these principles, then they would be deduced from something else and thus not innate. And in any case, many truths are known far earlier in life than such abstract principles — children know that “sweet is not bitter” long before they can articulate the principle of non-contradiction. The true explanation is simpler: these truths receive “general assent” not because they are innate but because “the mind, having once got the ideas, and compared them together, cannot but perceive their agreement.” The universality of certain truths, Locke concludes, is no proof of their innateness, and all the arguments offered for innate ideas collapse under examination.

This chapter establishes the method Locke will use throughout Book I: taking the strongest arguments for innatism and showing that they rest on confusion or faulty evidence. He is not merely denying a doctrine; he is insisting that any claim about the mind’s original furniture must be tested against actual observation of how minds work — particularly the minds of children, who are closest to the “state of nature” in which the mind begins.

Book I — Chapter 3: No Innate Practical Principles

Locke now turns from speculative principles (truths of logic and metaphysics) to practical principles — moral rules that some philosophers (notably Lord Herbert of Cherbury) had claimed were innate. If there were innate moral laws, Locke argues, they would have to be universally acknowledged. But the history of humanity shows the opposite: “There is scarce that principle of morality to be named, or rule of virtue to be thought on, which is not, somewhere or other, slighted and condemned by the general fashion of whole societies of men, governed by practical opinions and rules of living quite opposite to others.”

Locke provides numerous examples. The rule “parents, preserve and cherish your children” is violated by whole cultures that practice exposure of infants. “Justice, keep your promises” is violated by entire peoples who see nothing wrong with breaking faith with enemies. The duty to worship God is not recognized by some societies. “The taking away of the life of any man, but for some very great crime, is a thing condemned by the common light of nature,” Locke writes, yet “whole nations have been entertained with actions of blood and cruelty” where murder is not only permitted but praised. He is careful to note that he is not denying the reality of moral obligation — only that it is imprinted on the mind at birth independently of experience, reason, and social teaching.

Locke then addresses the claim that innate practical principles are “imprinted” but can be “blotted out” by custom and education. This, he says, is a contradiction: “If the same principle may be, by practice, worn out, or by custom or education, obliterated, then it is not an innate principle.” An impression that can be erased is no impression at all. He also anticipates the objection that conscience provides evidence of innate moral knowledge. Conscience, Locke responds, is “nothing else but our own opinion or judgment of the moral rectitude or pravity of our own actions.” Conscience varies between persons and cultures precisely because it is formed by education, custom, and reflection — not because it is a voice from birth.

The significance of this chapter is that Locke denies any special status to moral knowledge. Morality, like all knowledge, must be built up from experience and reason. There are no shortcuts, no inherent moral compass. This clears the ground for Locke’s own account of how moral ideas are formed — which will come much later, in Book IV.

Book I — Chapter 4: Other Considerations Concerning Innate Principles, Both Speculative and Practical

Locke consolidates his attack by addressing further arguments for innate ideas and offering a final diagnosis of why the doctrine has been so widely accepted. He considers the claim that the idea of God is innate — that every human being, by nature, has an idea of a supreme being. Locke points to the wide variety of religious beliefs across cultures, including societies that have “no notion of a God, no religion.” The idea of God, he argues, is “no more innate than the idea of a sun, or a star.” It must be acquired through observation of the world and reasoning about causes. Children and unreflective people often have no such idea until it is taught to them. Even where the idea exists, it differs radically between cultures — some conceive God as a spirit, others as material, others as a simple force. If the idea were innate, it would be uniform.

Locke also addresses the notion of innate truths about God — that God exists, that he is to be worshipped, that he will reward the good and punish the wicked. If these are innate, he asks, why do they need to be “proved” by reason? And why have so many human beings never arrived at them? “The true ground of that proposition (that there is a God) is not any innate impression, but the same that all other propositions have, the observation and experience of the works of God.” He acknowledges that the existence of God can be known with certainty through reason — but this is a demonstration, not an innate intuition.

The chapter closes with Locke’s diagnosis of the appeal of innate ideas. He argues that the doctrine has been maintained by philosophers and theologians because it gives them authority: “It is of no small power to make men those who receive it, believe that they are in possession of the truth, without further examination.” If moral and religious principles are innate, then no one need defend them by reason — they can simply assert them. Locke calls this “a short and easy way with all dissenters.” The doctrine of innate ideas, he concludes, is “a supposition that is not necessary” and, worse, “a supposition that is false.” With that, Book I ends and the constructive work of the Essay can begin.

This final chapter of Book I serves as both a rebuttal and a transition. Locke has cleared away the doctrine that would make experience irrelevant to knowledge. What remains is the task he set in the Introduction: to trace all ideas back to their origins in sensation and reflection, and to determine the extent and limits of what the understanding can know. That work occupies the remaining three books.

Book II — Of Ideas: Chapter I — Of Ideas in General, and Their Original

We enter Book II, where Locke shifts from the negative work of refuting innate principles to the positive task of tracing all ideas to their actual origins. This chapter lays the foundation for the entire theory of knowledge that follows.

Locke begins by defining his subject: an idea is “whatsoever is the object of the understanding when a man thinks.” This includes “phantasm, notion, species, or whatever it is which the mind can be employed about in thinking.” Every idea, he insists, comes from one of two sources. The first is sensation: our senses convey into the mind “distinct perceptions of things” such as yellow, white, heat, cold, soft, hard, bitter, sweet, and all those qualities we call sensible. The second is reflection: the mind observes its own operations — perceiving, thinking, doubting, believing, reasoning, knowing, willing — and from this observation obtains ideas that sensation could not supply.

“Let us then suppose the mind to be, as we say, white paper, void of all characters, without any ideas: — How comes it to be furnished? Whence comes it by that vast store which the busy and boundless fancy of man has painted on it with an almost endless variety? Whence has it all the materials of reason and knowledge? To this I answer, in one word, from EXPERIENCE.”

Locke then introduces the famous metaphor of the tabula rasa — the mind as white paper — and declares that all ideas derive from experience. Sensation provides ideas of external objects; reflection provides ideas of the mind’s own internal operations. These two, he says, are “the fountains of knowledge, from whence all the ideas we have, or can naturally have, do spring.”

A critical distinction follows: the mind is passive in receiving simple ideas from sensation and reflection, but active in combining, comparing, and abstracting them into complex ones. Children acquire ideas gradually: the first ideas come through sensation (the nurse’s nipple, the mother’s face), and only later, when the mind turns to reflect on its own operations, do ideas of thinking and willing arise. Locke notes that reflection appears later because it requires attention, and children are too occupied with sensation to notice the mind’s own activity. Yet all ideas without exception come from one or both of these sources.

This chapter establishes the entire explanatory framework for what follows. Locke’s claim is absolute: no idea enters the understanding except through sensation or reflection. The mind is not born with furniture; it is born empty, and experience fills it. This position directly opposes the innate doctrine of Book I and commits Locke to a thoroughgoing empiricism — any idea we claim to have must be traceable back to one of these two sources, or it is not a genuine idea at all.

Book II — Chapter 2: Of Simple Ideas

Having established that all ideas come from experience, Locke now examines the simplest elements of thought. Simple ideas are “the materials of all our knowledge” — they enter the mind pure and unmixed, and the mind cannot create or destroy them, only combine and arrange them.

Locke explains that simple ideas come into the mind through one sense, through multiple senses, or through reflection alone. Those that come through one sense alone include colors (sight), sounds (hearing), tastes (taste), smells (smell), and tangible qualities such as heat, cold, and solidity (touch). These ideas are “perfectly distinct” from one another — the idea of blue is not the idea of sweet, and the mind cannot exchange them.

“These simple ideas, the materials of all our knowledge, are suggested and furnished to the mind only by those two ways above mentioned, viz. sensation and reflection.”

Ideas that come through more than one sense include space, extension, figure, rest, and motion — these are perceived both by sight and touch. Ideas of reflection alone include perception (the mind’s awareness of its own operations) and willing (the mind’s power to initiate or direct its actions). Both sensation and reflection jointly produce ideas such as pleasure, pain, power, existence, and unity — these arise from the mind’s observation of external objects and internal operations alike.

The crucial property of simple ideas is that the mind cannot invent them. A blind man cannot form an idea of color; a deaf man cannot form an idea of sound. The mind can combine, repeat, and compare the simple ideas it has received, but it cannot fabricate a new simple idea from nothing. As Locke puts it, “the dominion of man” over the world of ideas is limited to compounding and dividing the materials that sensation and reflection provide.

This chapter establishes the atomic theory of ideas that structures the rest of Book II. Simple ideas are the atoms of thought — indivisible, given, and beyond the mind’s power to create. All the vast complexity of human knowledge is nothing but these simple elements combined in various ways. The task of the philosopher, then, is to analyze complex ideas back into their simple components, thereby discovering whether they have legitimate empirical foundations or are mere chimeras of the imagination.

Book II — Chapter 3: Of Ideas of One Sense

Locke now examines the simple ideas that enter through a single sense, beginning with sight. Here he introduces an important distinction that will recur throughout the Essay: the difference between the idea itself and the quality in the object that produces it. The mind does not perceive objects directly; it perceives only the ideas that objects produce in the mind through sensation. The qualities in objects that produce these ideas are powers — the power of a rose to produce the idea of red, the power of a bell to produce the idea of sound.

Locke lists the ideas that come through sight: colors in all their varieties — white, red, yellow, blue, green, and all the shades between — along with light itself. Through hearing: sounds — notes of music, voices, noise. Through smell: odors — fragrant, stinking, sweet, sour. Through taste: flavors — sweet, bitter, sour, salt, and all the complex tastes of food and drink. Through touch: heat, cold, solidity, hardness, softness, roughness, smoothness, and other tangible qualities.

“If it were possible for a man born blind to have his sight restored, he would not at first have any idea of the distance, figure, or magnitude of the objects he saw.”

Locke pauses to reflect on a particular question: whether a blind man, upon gaining sight, could recognize shapes by sight that he previously knew only by touch. His answer is no — the ideas of shape from sight and touch are entirely different sensations, connected only by experience. A sphere and a cube, known by touch, would not be recognized by sight until the man learned to associate the visual ideas with the tactile ones. This observation, famous in the history of philosophy, illustrates Locke’s commitment to the radical empiricism that all ideas are learned through experience — even the seemingly automatic connection between sight and touch is a product of habit and association.

The chapter matters because it establishes that each sense provides its own unique set of simple ideas, and that these ideas are incommensurable — the idea of red cannot be translated into the idea of warmth, nor the idea of a sound into the idea of a taste. The mind’s unity is not given by a pre-existing harmony of the senses but must be built up through the repeated experience of objects that affect multiple senses simultaneously. This sets the stage for Locke’s later treatment of complex ideas, where the mind actively combines these disparate sensory inputs into unified representations of objects.

Book II — Chapter 4: Of Solidity

This chapter addresses a single but crucial simple idea — solidity — which Locke argues is the most fundamental idea we receive from touch. Solidity is the idea of a body’s filling space, of its being impenetrable, of its resisting the approach of other bodies. It is “the idea of the cohesion of the parts of matter” and is essential to the very concept of body.

Locke distinguishes solidity from hardness: hardness is the firm cohesion of a body’s parts so that they do not easily yield to pressure, while solidity is the simple filling of space that makes any body impenetrable. A soft body (such as water) still has solidity — it still occupies space and resists being compressed into a smaller volume. Hardness is a relative degree of the cohesion of parts; solidity is absolute.

“This is the idea of body, whereby we conceive it to be a thing that fills space, or is impenetrable, and that hinderedth the approach of two bodies when they are moving towards one another.”

Locke argues that solidity is essential to our distinction between body and space. Pure space, or extension, is the idea of distance between bodies without any resistance. Body combines extension with solidity — it takes up space and resists the entry of other bodies into that space. Without solidity, we would have no way to distinguish matter from empty space, or bodies from the shadows and illusions that appear to occupy space without resisting.

Locke also connects solidity to the idea of body as distinct from spirit or mind. Body is that which is extended and solid; spirit is that which thinks and wills. These two substances are known only through their respective sets of simple ideas — body through solidity, extension, figure, motion; spirit through perception, thinking, doubting, willing. Neither is reducible to the other, and both are known only through the ideas they produce in us.

This chapter performs an important philosophical function: it grounds the concept of material substance in a specific, empirically-derived simple idea. Solidity is not an a priori concept or an innate notion; it is an idea that enters the mind through the sense of touch, and it provides the empirical basis for everything we say about bodies. Locke is showing how even the most fundamental categories of physics — matter, body, space — can be traced back to simple sensory origins.

Book II — Chapter 5: Of Simple Ideas of Divers Senses

Locke now treats a small group of simple ideas that enter through more than one sense. These are space, extension, figure, rest, and motion. Both sight and touch convey these ideas: we see the shape of a cube, and we can also feel its shape; we see motion across a visual field, and we can feel the motion of a hand passing over our skin.

“The ideas we get by more than one sense are of space or extension, figure, rest, and motion: for these make perceivable impressions, both on the eyes and touch.”

Locke’s point here is that these ideas have a special status: they are not unique to any single sense organ but are common to two of our primary channels of sensation. This gives them a kind of cross-modal generality — they are not peculiar to sight alone or touch alone but represent features of the world that both senses can detect. A blind man can know the shape of a cube through touch; a sighted man can know it through sight. The idea of shape, therefore, is not dependent on any particular sensory modality.

This cross-modal availability is important for Locke’s theory of knowledge because it explains how we can have ideas of objects that are stable across different sensory experiences. When we see and touch the same apple, the ideas of its shape, size, and motion are consistent across both senses, reinforcing our sense of encountering a single external object. The ideas that come through only one sense — color, sound, taste — are more easily treated as secondary qualities, subjective and dependent on the particular sense organ.

Locke is laying the groundwork here for his later distinction between primary and secondary qualities. The ideas available to multiple senses (extension, figure, motion, rest, solidity) will become the primary qualities — properties that really exist in bodies themselves. The ideas available only to a single sense (color, sound, taste, smell) will become secondary qualities — powers in bodies to produce sensations in us, but not resembling anything in the bodies themselves.

Book II — Chapter 6: Of Simple Ideas of Reflection

Locke now turns to the second fountain of ideas: reflection, the mind’s observation of its own operations. The simple ideas of reflection are those the mind gets by “turning in upon itself” and observing its own activities. The two principal operations the mind observes are perception (which Locke also calls thinking) and volition (or willing).

Perception is “the first faculty of the mind” and is the simplest idea of reflection. It is the mind’s awareness of its own ideas — the consciousness that accompanies every act of thinking. Locke insists that perception is distinct from sensation: sensation brings ideas into the mind from outside, but perception is the mind’s awareness of those ideas once they arrive. Without perception, sensation would be meaningless — a blind man’s eyes receive light, but without perception, no idea of color arises.

“Perception, as it is the first faculty of the mind, so it is the first and simplest idea we have from reflection.”

Volition, or willing, is the second simple idea of reflection. The mind observes itself initiating actions — deciding to move a limb, choosing to think about one subject rather than another, determining to pursue a goal. This idea of power — the power to begin or forbear, continue or end, certain actions of the mind and body — is derived from reflection on the mind’s own experience of choosing.

Locke also notes that pleasure and pain — in the broad sense of delight and uneasiness — accompany almost all our sensations and reflections. We delight in certain perceptions and are pained by others; we find pleasure in some operations of the mind and uneasiness in others. These ideas of pleasure and pain are simple ideas that come from both sensation and reflection, and they form the basis of our concepts of good and evil (good being what produces pleasure, evil what produces pain).

This chapter completes Locke’s inventory of the sources of simple ideas. Sensation provides ideas of external objects; reflection provides ideas of the mind’s own operations. Together, these two fountains supply all the materials of human knowledge. The next chapters will examine more complex phenomena — memory, retention, discerning, comparing — and show how the mind operates upon its simple ideas to produce the elaborate structures of thought.

Book II — Chapter 7: Of Simple Ideas of Both Sensation and Reflection

Locke examines a set of simple ideas that arise from both sensation and reflection simultaneously. These include pleasure and pain, power, existence, and unity. Each of these ideas arrives in the mind through multiple channels.

Existence, for instance, is suggested to the mind by every sensation (I perceive something — a color, a sound — and thereby have the idea of that thing existing) and by every reflection (I think, and thereby have the idea of myself existing as a thinking being). Unity is similarly universal: every idea, whether from sensation or reflection, is accompanied by the idea of oneness — each idea is one idea, each object is one object.

“Pleasure and pain, and that which causes them, — delight and uneasiness, — are the hinges on which our passions turn.”

Pleasure and pain Locke treats with particular attention. They are not merely sensations in the narrow sense but are “the hinges on which our passions turn.” The desire for pleasure and aversion to pain are the motives behind all voluntary action. What we call love is the idea of pleasure associated with an object; what we call hatred is the idea of pain associated with an object. Joy is pleasure arising from the thought of a present good; sorrow is pain from the thought of a present evil. Hope is pleasure from the expectation of future good; fear is pain from the expectation of future evil. Locke systematically derives the whole range of human passions from these two simple ideas.

Power is an idea obtained from both sensation — we see bodies moving and affecting one another, and we infer that they have the power to cause change — and reflection — we experience ourselves choosing to act and refrain from acting. The idea of power thus has a dual origin, which will become important when Locke later discusses free will and moral responsibility.

This chapter serves to demonstrate that even the most abstract and universal ideas — existence, unity, power — have empirical origins. They are not innate principles that the mind brings to experience but are derived from experience itself. The mind does not come equipped with the concept of existence; it gradually forms that concept by observing that the objects of its sensations and reflections are present to it. Locke is systematically dismantling the rationalist position that certain ideas are a priori necessary conditions of experience, and replacing it with a thoroughgoing empiricism that derives all ideas from particular experiences.

Book II — Chapter 8: Some Further Considerations Concerning Our Simple Ideas of Sensation

We are now in Book II, Chapter 8, where Locke introduces a distinction that will become foundational for his entire account of perception and qualities. This chapter classifies the qualities that produce simple ideas in us and establishes the critical distinction between primary and secondary qualities.

Locke begins by noting that the mind is “wholly passive” in receiving simple ideas from sensation and reflection. These ideas “force themselves upon the mind” and cannot be refused, altered, or obliterated by the understanding. Yet the objects that produce these ideas in us do so through their qualities — the powers objects have to produce ideas in our minds. Locke defines a quality as “the power to produce any idea in our mind.” He then divides qualities into two kinds: primary qualities and secondary qualities.

Primary qualities are “utterly inseparable from the body, in what state soever it be.” These include solidity, extension, figure, motion or rest, and number. These qualities are “in the things themselves, whether they are perceived or no.” A grain of wheat, divided repeatedly, retains solidity, extension, figure, and mobility in each of its parts. These qualities are “real” or “original” because they exist in the body itself and produce simple ideas that resemble them. When we perceive the roundness of a ball, the idea of roundness resembles the actual figure of the ball.

“The ideas of primary qualities of bodies are resemblances of them, and their patterns do really exist in the bodies themselves.”

Secondary qualities, by contrast, are “nothing in the objects themselves but powers to produce various sensations in us by their primary qualities.” These include colors, sounds, tastes, smells, and tangible qualities such as heat and cold. These qualities produce ideas in us that have no resemblance to anything in the objects themselves. The idea of redness or sweetness does not resemble any actual quality in the apple; rather, the apple’s primary qualities — its bulk, texture, and motion of its parts — are arranged in such a way as to produce those sensations in us. Locke offers the example of fire: “The fire has a power to produce in us a new idea of pain or of pleasure” — but the pain is not in the fire.

Locke clarifies that there is also a third kind of quality, which he calls powers: the capacity of one body to alter the primary qualities of another body, such as fire’s power to melt wax. These are distinct from secondary qualities because the effect is a change in the object itself rather than a sensation in the perceiver. However, Locke groups these powers under the same general class as secondary qualities, since both depend on the primary qualities of bodies for their operation.

This chapter establishes that while primary qualities produce ideas that resemble their causes, secondary qualities produce ideas that are merely effects of the arrangement of primary qualities. The world in itself consists only of bodies with primary qualities; the colors, sounds, and tastes we perceive are subjective additions of the mind. Locke is careful to note that “the ideas of secondary qualities are also real” in the sense that they are produced by real powers in objects, but they do not correspond to anything in the objects that resembles them.

Book II — Chapter 9: Of Perception

We move now to Chapter 9, where Locke examines perception as the first and simplest faculty of the understanding. Perception is the act by which the mind receives ideas, and it is “the first step and degree towards knowledge, and the inlet of all the materials of it.”

Locke defines perception as “the perception of the operations of our own minds within us, as it is employed about the ideas it has got.” It is “the first faculty of the mind exercised about our ideas.” Perception, he insists, is a passive faculty in the sense that the mind cannot help but perceive when ideas are presented to it, but it is also active in the sense that the mind attends to some perceptions and ignores others. The infant receives light and colors as soon as the eye opens, but does not understand them at first.

Locke distinguishes perception from thinking or contemplation. Perception is the mere reception of ideas; thinking involves comparing, combining, and abstracting ideas. Perception is also distinct from sensation, which is the act of the bodily organs receiving impressions from external objects. Perception is the mind’s awareness of those impressions.

“Perception is the first operation of all our intellectual faculties, and the inlet of all knowledge into our minds.”

Locke observes that perception is accompanied by judgment in most cases. When we see a globe, the actual idea presented to the eye is that of a flat circle of varying shades, but the mind, through experience, judges it to be a round sphere. This judgment occurs so quickly and habitually that we mistake it for direct perception. Both children and adults receive the same impressions from external objects, but adults have learned to interpret those impressions through repeated experience.

This chapter also addresses perception in animals. Locke argues that animals have perception, as is evident from their behavior — they avoid pain, seek pleasure, and respond to their environment. But he maintains a distinction between perception and the higher faculties of reason and abstract thought, which animals lack. Perception is shared across creatures; the capacity to abstract and compare ideas is distinctively human.

Locke emphasizes that perception is the foundation of all knowledge. Without perception, “all other faculties of the mind would be to no purpose.” It is through perception that the mind receives the materials of thought, and it is through reflection on those perceptions that the mind builds all its complex ideas and knowledge.

Book II — Chapter 10: Of Retention

Chapter 10 examines the mind’s ability to retain ideas after they have been perceived. Locke identifies two faculties of retention: contemplation and memory.

Contemplation is the ability to keep an idea present to the mind for a period of time after the original sensation has ceased. Locke describes it as “the keeping of an idea in the mind, when the object that first caused it is removed.” If one looks at a candle and then closes their eyes, they can still hold the image of the flame in their mind for a short while. This is the simplest form of retention.

Memory is the more important faculty: “the power to revive again in our minds those ideas which, after imprinting, have disappeared, or have been as it were laid aside out of sight.” Memory differs from contemplation in that it involves recalling ideas that have been absent from consciousness for some time. Memory is “the storehouse of our ideas,” where ideas are kept even when the mind is not actively thinking of them.

Locke notes that memory is essential to all other intellectual operations. Without memory, “we should have no knowledge of things beyond our present sensation.” Animals, he observes, also possess memory — a dog that has been beaten fears the stick because it remembers the pain. But human memory, combined with reason, enables the complex operations of comparing, combining, and abstracting ideas.

“Memory is the power to revive again in our minds those ideas which, after imprinting, have disappeared.”

Locke discusses various phenomena of memory: the way ideas fade over time, the difficulty of recalling some ideas while others are easily retrieved, and the phenomenon of forgetting. He observes that ideas that are frequently repeated or attended to with “attention and pleasure” are most easily remembered. Memory also depends on the physical state of the brain: “the memory of some men, it is true, is very tenacious, even to a miracle; but yet there seems to be a constant decay of all our ideas.”

Locke addresses the question of whether memory is essential to personal identity — a foreshadowing of his famous discussion in Book II, Chapter 27. He notes that “personal identity” depends on consciousness, and memory is what allows consciousness to extend backwards and connect past experiences to the present self. But he postpones the full discussion of this topic.

This chapter establishes that retention is a necessary supplement to perception. Without the ability to retain ideas, perception would be momentary and useless for knowledge. The mind would be like a mirror that reflects images but retains nothing when the object is removed. Memory enables the mind to accumulate experience and to draw on past perceptions to inform present judgments.

Book II — Chapter 11: Of Discerning, and Other Operations of the Mind

Chapter 11 examines the active operations of the mind upon the ideas that perception and memory have supplied. Locke identifies several distinct faculties or operations that transform simple ideas into complex ones.

The first and most fundamental operation is discerning or distinguishing ideas from one another. This is “the first degree of thought” and is necessary for all judgment. Without the capacity to distinguish one idea from another, we could not perceive difference or sameness, and all knowledge would be impossible. Locke notes that “clearness and distinctness” are properties of ideas that enable this operation — clear ideas are those that present themselves vividly to the mind, distinct ideas are those that are sharply separated from other ideas.

The second operation is comparing ideas with one another. Locke calls this “the first step towards the discovery of relations.” By comparing ideas, the mind notices agreements and disagreements, which form the basis of judgment. Comparison is the foundation of all relatio, including cause and effect, identity, and diversity.

The third operation is compounding or composing simple ideas into complex ones. This is the process by which the mind creates complex ideas of substances, modes, and relations. Locke describes it as “the putting together of several simple ideas to make one compound one.” For example, the simple ideas of whiteness, hardness, sweetness, and a particular shape are compounded to form the complex idea of a sugar cube.

“The mind has a power to consider several of its ideas united together in a reasonable order, and so to make the compound idea of a mode or a substance.”

The fourth and most distinctively human operation is abstraction. Abstraction is “the separating of a particular idea from all other ideas that accompany it in its real existence.” It is the process by which we form general ideas from particular ones. For example, from the particular idea of a white horse, the mind can abstract the general idea of “whiteness” or “horse.” Locke argues that animals do not possess the power of abstraction, and this is what separates human understanding from brute perception. Children learn to abstract gradually, and those who lack this power remain at the level of mere perception of particulars.

Locke also discusses naming as an operation related to abstraction. General ideas are tied to general words, which serve as signs for classes of things. Without language, abstraction would be limited, and without abstraction, we could have no general knowledge.

This chapter presents the active powers of the understanding as a progressive series: discerning, comparing, compounding, and abstracting. These operations build upon the passive reception of simple ideas and transform them into the materials of knowledge. Locke is careful to emphasize that all of these operations work only on ideas that have been received through sensation and reflection; the mind does not create any new simple ideas, but only rearranges and combines those it has received.

Book II — Chapter 12: Of Complex Ideas

Chapter 12 provides a systematic classification of all complex ideas. Locke defines complex ideas as those made by the mind “by putting together several simple ideas, or by combining simple ideas.” He identifies three kinds of complex ideas: modes, substances, and relations.

Modes are complex ideas that do not contain within themselves the supposition of an independent existence. They are “such complex ideas which, however compounded, contain not in them the supposition of subsisting by themselves.” Modes are always considered as affections of substances. Locke distinguishes simple modes (variations of the same simple idea, such as a dozen or a score, which are variations of number) from mixed modes (combinations of different kinds of simple ideas, such as beauty, theft, or gratitude).

Substances are complex ideas that represent “particular things subsisting by themselves” — they are “the supposed but unknown support of those qualities we find existing.” The complex idea of a substance combines simple ideas of qualities that are observed to coexist (whiteness, hardness, sweetness in sugar) and adds the obscure idea of a “substratum” or “support.” Substances include single substances (man, horse, gold) and collective substances (army, flock, fleet).

Relations are complex ideas that arise from “comparing one idea with another” and noticing agreements, disagreements, or connections between them. Relations include cause and effect, identity and diversity, time and place, proportion, and moral relations.

“The mind, by the power it has over its ideas, can put them together, and thus make complex ideas of modes, substances, and relations.”

Locke notes that all complex ideas, however abstract or elaborate, are ultimately composed of simple ideas derived from sensation and reflection. The idea of “gratitude,” for example, is a mixed mode composed of simple ideas of pleasure, the recognition of a benefit received, and a disposition to return the kindness. The idea of “god” is a complex idea formed by “enlarging” the simple ideas of existence, duration, knowledge, power, and pleasure to infinity.

This chapter provides the framework for all subsequent discussions in Book II. Having classified all ideas into simple and complex, and having divided complex ideas into modes, substances, and relations, Locke proceeds to examine each category in detail. The classification is exhaustive — every idea that can enter the human understanding falls into one of these categories. The mind is limited to the materials supplied by experience, but within those limits it has immense creative power to combine, compare, and abstract.

Book II — Chapter 13: Of Simple Modes, and First of the Simple Modes of Space

Chapter 13 begins the detailed examination of simple modes by considering the simple modes of space. Locke introduces the idea of space through the senses of sight and touch, and then considers its modes: distance, capacity, and extension.

The simple idea of space is conveyed to the mind through the perception of bodies that are separated from one another. When we see a body at a distance from another, we form the idea of the space between them. Locke calls this idea “the idea of space” and describes it as “that which is between bodies.” This idea is then distinguished from the bodies themselves — space is not a body, but is “that which contains bodies.”

Locke examines the simple modes of space: distance (the space between two points), capacity (the space contained within a body), and extension (the length, breadth, and depth of space). The mind can combine these simple modes in infinite ways, producing the ideas of a foot, a yard, a mile, or a league. By repeating the simple idea of a fixed length, the mind can form the idea of any determinate space.

“The idea of space is conveyed to the mind by the sight and touch; it is the idea of that which is between bodies.”

Locke also discusses the relationship between space and place. Place is “the relative position of anything” — the location of a body with respect to other bodies. Place depends on the idea of space, but adds the notion of position or situation. A body’s place is determined by its distance from certain fixed reference points.

A distinctive feature of this chapter is Locke’s treatment of vacuum or void space. He argues that we have a clear idea of space without body — “a man may conceive space without body.” This idea of void space is not merely negative; it is a positive idea of extension without solidity. Locke defends the possibility of a vacuum against those who, following Descartes, argued that extension is identical with body. For Locke, space and body are distinct: space is extension considered as empty of body, while body is extension combined with solidity.

Locke also examines solidity (discussed earlier in Chapter 4) as distinct from space. Solidity is the power of bodies to resist penetration; space has no such power. A body occupies a portion of space, but space itself offers no resistance. The distinction between space and solidity is fundamental: space is “perfectly penetrable” while body is “not penetrable.”

This chapter establishes that space is a simple idea whose modes can be combined in infinite ways. The idea of space is derived from sensation, but the mind can manipulate its modes freely — we can conceive of distances far beyond anything we have actually perceived, and we can imagine space extending infinitely in all directions. The simple modes of space provide the framework for all quantitative measurement and geometry.

Book II — Chapter 14: Of Duration and Its Simple Modes

We are in Book II, Chapter 14, where Locke turns from the simple modes of space to examine the simple modes of another fundamental idea derived from sensation and reflection: duration.

Locke begins by distinguishing duration from the idea of time. The idea of duration comes from reflection on the succession of ideas in our own minds. As we observe one idea follow another, we acquire the idea of succession, and from this succession we derive the idea of duration. Duration is “fleeting extension” — a kind of extension in a single line, whose parts are not permanent but pass away in succession. This is fundamentally different from space, whose parts are co-existent and permanent. Locke writes:

“It is evident to any one who will but observe what passes in his own mind, that there is a train of ideas which constantly succeed one another in his understanding, as long as he is awake. Reflection on these appearances of several ideas one after another in our minds, is that which furnishes us with the idea of succession: and the distance between any parts of that succession, or between the appearance of any two ideas in our mind, is that we call duration.”

From the idea of duration, the mind obtains the simple modes of duration by repeating and compounding the idea. The most important of these modes is time — which Locke defines as duration set out and measured by the motion of observable bodies, particularly the sun and stars. Time is “duration set out by measures” — it is duration made useful and measurable by applying standard periods such as hours, days, and years. The mind can also conceive of eternity by repeating the idea of duration without end, though this stretches the finite mind to its limits. Locke notes that the idea of eternity is simply the idea of “infinite duration” — a repetition of the idea of any length of duration, repeated again and again, without ever stopping.

This chapter establishes that duration, like space, is a simple idea whose modes are constructed by the mind through repetition. The idea of time is not an innate notion but a complex construction from the simple idea of duration, combined with observations of regular motion. The distinction between duration and time is crucial: duration is the pure idea of succession itself; time is duration as measured by motion. Our idea of eternity is a negative idea — we know what it is not (limited) rather than positively comprehending its infinite nature.

Book II — Chapter 15: Of Duration and Expansion, Considered Together

In Chapter 15, Locke brings together the two fundamental extensions — duration and expansion (space) — comparing them directly and exploring their relationship.

Locke observes that both duration and expansion are species of the same general idea: extension. But they differ in crucial respects. Expansion is “the extension of bodies in length, breadth, and thickness” — its parts are co-existent, fixed, and can be considered all at once. Duration, by contrast, has only one dimension — a continuous line of succession — and its parts are never all present together; they flow one after another. The idea of distance applies to both: we speak of distance in space (miles) and distance in time (hours). But spatial distance is measured between co-existing points, while temporal distance is measured between successive events. Locke emphasizes that the idea of space is “a capacity of receiving bodies” — it is about co-existence and position — while the idea of duration is about “the distance and succession of ideas” in the mind.

A particularly important distinction Locke makes is between the immensity of space and the eternity of duration. Immensity is the idea of space without bound — extension that never comes to an end. Eternity is the idea of duration without bound — succession that never begins and never ends. Both are positive ideas of endlessness, but they apply to different dimensions of experience. Locke notes that we cannot positively conceive of either immensity or eternity — we can only approach them through the negative idea of “no end” or “no beginning.” Our finite minds can repeat the ideas of space and duration indefinitely, but we never actually grasp the infinite in a positive concept.

This chapter clarifies that space and time are parallel but distinct frameworks for ordering our ideas. Both are simple modes derived from sensation and reflection, and both can be extended in imagination beyond any actual experience. But they remain separate ideas: space orders co-existence, time orders succession. The confusion between them arises because we use spatial language to talk about time (e.g., “a long time,” “a short time”), which Locke warns against as a source of confusion.

Book II — Chapter 16: Of Number

Chapter 16 examines number as one of the simplest and most important simple modes of sensation.

Locke begins by declaring that number is “the most simple and most universal idea” — it is applicable to everything that exists or can be conceived. The idea of number is acquired through sensation and reflection, arising from the perception of unity. Every object of sensation or reflection is one thing — an individual idea. From the perception of unity, the mind gains the idea of one. By repeating this idea, the mind constructs all numbers: two is one and one, three is one and one and one, and so on. Number is therefore a simple mode formed by the repetition of the simplest of all ideas — the idea of unity itself. Locke writes:

“Number applies itself to men, angels, actions, thoughts — every thing that either doth exist or can be imagined. By the repetition of the idea of an unit, and joining it to another unit, we make thereof one collective idea, marked by the name two. And whosoever can do this, and proceed on, adding a unit to the last collective idea which he has of any number, and giving a name to it, may count, or have ideas, for several collections of units.”

Locke emphasizes that number is the idea on which all other ideas of relation and proportion depend. The certainty of mathematics rests on the clear and distinct ideas of number — we can be absolutely certain that two plus two equals four because the ideas are perfectly clear and the operations are transparent to the mind. Number also provides the clearest example of demonstrative knowledge: we can prove mathematical truths by showing the connections between ideas of number. Unlike space and duration, number has no degree — it cannot be more or less in the same way that heat or cold can be more or less. A number is exactly what it is: three is three and cannot be two.

This chapter establishes number as the foundational simple mode for all quantitative reasoning. Because the idea of unity is so simple and universal, number applies to every domain of human thought. The clarity and certainty of arithmetic provides the paradigm for demonstrative knowledge: when we understand the relations between numbers, we have knowledge that is both certain and universal, independent of any particular experience.

Book II — Chapter 17: Of Infinity

In Chapter 17, Locke investigates the idea of infinity, beginning with its origins and then examining how it applies to space, duration, and number.

Locke argues that the idea of infinity is not a positive idea but a negative idea — it is the idea of endlessness, of “never coming to an end.” The idea arises because the mind can repeat any finite idea indefinitely. When we take any length of space or duration, we can always imagine adding more — the mind never reaches a necessary stopping point. Infinity is simply “the supposed endless progression” of finite ideas. Locke is careful to distinguish between the idea of infinite space and the idea of an infinite number: we can conceive of infinite space (space without end) but we cannot conceive of infinite number (number that includes all numbers), because number is essentially the idea of a determined quantity, and an infinite number would be a contradiction. He writes:

“Whatever positive ideas we have of space, duration, or number, they are all finite, and consist of parts that are finite. But when we suppose an inexhaustible remainder, from which we remove all bounds, and which we cannot come to an end of, we have that negative idea of infinite.”

Locke applies this analysis to the idea of God. Our idea of God includes the attribute of eternity (infinite duration) and immensity (infinite space). But these ideas are negative: we do not positively conceive of what God’s eternity or immensity is; we only conceive that it has no limits. Our idea of God’s infinity is therefore “the negation of an end” — we know what it is not, rather than positively knowing what it is. This is not a defect in our idea of God, Locke insists — it is the nature of finite minds to grasp the infinite only negatively. We can be certain that God is infinite without having a positive idea of what that infinity consists in.

This chapter establishes that infinity is not a simple idea derived from sensation or reflection but a complex idea built by the mind’s power of repetition. The infinite is not something we perceive; it is something we construct by pushing finite ideas beyond any actual limit. The distinction between positive and negative ideas of infinity is crucial: we have clear ideas of finite space and finite duration, but infinity is always a “negative” idea — the idea of no end, no limit — which is perfectly adequate for our purposes though not fully positive.

Book II — Chapter 18: Of Other Simple Modes

Chapter 18 briefly covers several remaining simple modes that Locke has not yet discussed: those of motion, sound, colors, tastes, and smells.

Locke begins by noting that the simple modes of motion are obtained by considering the degrees or varieties of motion. A body can move fast or slow, in a straight line or in a curve, in one direction or another. Each variety of motion is a simple mode of the idea of motion, which itself is a simple idea from sensation. The mind can combine these modes in various ways, though the combinations are constrained by the nature of bodies and forces. For example, we can conceive of a body moving in a circle at a uniform speed — this is a simple mode of motion. We can conceive of accelerating or decelerating motion — also simple modes.

For the secondary qualities — colors, sounds, tastes, smells — the simple modes are less numerous and less capable of precise combination. Colors have simple modes like “red, blue, green, yellow” and their various shades and mixtures. But Locke notes that the simple modes of these qualities are not as systematic as those of space, duration, or number. We do not have clear and distinct names for all the variations of color or taste; we rely on comparisons to known objects (“rose-red,” “lemon-yellow”). The mind can combine these simple modes into complex ideas (like the taste of a specific dish or the color of a particular flower), but the combinations are not governed by the same kind of necessary relations as the modes of number or extension.

This short chapter serves as a catch-all for the remaining simple modes. It emphasizes that while some simple ideas (space, duration, number) have highly systematic modes that can be precisely named and combined, others (colors, tastes) have modes that are more vague and less precisely articulated. The difference reflects the nature of the ideas themselves: those derived from primary qualities (solidity, extension, motion) are more amenable to precise measurement and combination than those derived from secondary qualities (colors, sounds, tastes).

Book II — Chapter 19: Of the Modes of Thinking

In Chapter 19, Locke examines the modes of thinking — the various ways in which the mind exercises its power of perception and reflection.

Locke begins by observing that the principal mode of thinking is perception, which is the first and simplest act of the understanding. But there are many other modes: remembrance (the recalling of past ideas), contemplation (the fixing of the mind on an idea for a length of time), attention (the focusing of the mind on a particular idea while others are present), study (the intense and prolonged application of the mind to a subject), meditation (the voluntary turning of the mind’s eye toward an idea), and reasoning (the perception of the agreement or disagreement of ideas). All of these are modes of thinking — different ways in which the simple idea of thinking is modified. Locke writes:

“The mind has a power to suspend the execution and satisfaction of any of its desires, and so is at liberty to consider the objects of them, examine them on all sides, and weigh them against others. This is the source of all liberty in thinking, as well as in action.”

Locke devotes particular attention to willing and volition as modes of thinking. When the mind actively determines itself to think of one thing rather than another, or to pursue one course of action rather than another, it exercises the power of volition. This is different from simple perception, which is often passive. Volition involves a deliberate choice — the mind actively selecting which ideas to consider or which actions to perform. The mode of thinking called desire is also a mode of thinking, involving the uneasiness of absence and the inclination toward a perceived good.

This chapter establishes that thinking is not a simple, uniform activity but comprises many distinct modes. Each mode is a modification of the simple idea of thinking, which comes from reflection. The distinction between passive perception and active volition is crucial for Locke’s philosophy of mind: we do not merely receive ideas; we actively manipulate them, choose among them, and direct our attention. This power of the mind to suspend, examine, and choose is the foundation of human freedom and moral responsibility.

Book II — Chapter 20: Of Modes of Pleasure and Pain

Chapter 20 introduces the ideas of pleasure and pain as simple ideas derived from sensation and reflection, and then examines their various modes.

Locke begins by stating that pleasure and pain are “simple ideas” that accompany almost every other idea. Whenever we perceive something that agrees with our nature or contributes to our well-being, we feel pleasure; whenever we perceive something that disagrees with our nature or threatens harm, we feel pain. Pleasure and pain are not qualities of objects themselves but are “the perception of the good or evil” that objects cause in us. They are the hinges on which all our passions turn. Locke writes:

“Pleasure and pain are simple ideas, and that which is the subject of them is the mind, as the organ of perception. For, whether we call it satisfaction, delight, happiness, or what you please, it is the same idea; and whether we call it uneasiness, trouble, pain, torment, anguish, misery, it is still the same.”

From these simple ideas, the mind constructs the complex modes of the passions or emotions: love is the idea of pleasure in the presence of its cause; hate is the idea of pain or aversion in the presence of its cause; desire is the uneasiness of the absence of a perceived good; joy is the pleasure arising from the possession of a good; sorrow is the pain arising from the loss of a good; hope is the pleasure of expecting a future good; fear is the uneasiness of expecting a future evil. Each passion is a complex mode formed by combining the simple idea of pleasure or pain with other ideas (presence/absence, certainty/uncertainty, past/future).

This chapter establishes that all human emotions and passions are ultimately modifications of the simple ideas of pleasure and pain. There is no passion that does not involve either pleasure or pain, or the expectation of one or the other. This does not reduce all human motivation to hedonism — it simply analyzes the structure of the passions. The modes of pleasure and pain are crucial for understanding human action, because the mind is always motivated to seek pleasure and avoid pain. Locke’s analysis here provides the foundation for his moral psychology.

Book II — Chapter 21: Of Power

Chapter 21 is one of the longest and most important in the entire Essay, examining the idea of power and its role in human action, liberty, and moral responsibility.

Locke begins by stating that the idea of power arises from reflection on the changes we observe in objects and in our own minds. We notice that one body can produce change in another (fire melts wax), and that our own minds can produce changes in our bodies and in our thoughts. The idea of power is the idea of “the ability to make, or to receive, any change.” Locke distinguishes two kinds of power: active power, which is the ability to produce change, and passive power, which is the ability to receive change. The clearest idea of active power comes from reflection on our own minds: we are aware that we can move our bodies, direct our thoughts, and make choices. This is the foundation of our idea of free will.

Locke then launches into his famous analysis of liberty and necessity. Liberty is “the power to act or not to act according to the determination of the mind.” A free agent is one who has the power to do or forbear an action, as he thinks fit. Necessity is the absence of this power — when an action is determined by external causes and the agent cannot do otherwise. Locke insists that liberty is not a property of the will but of the person. The will is simply “a power to choose” — it always follows the last judgment of the understanding about what is good or evil. The question of determinism is not about whether the will is free but about whether the person is free to act on their choices. He writes:

“Liberty, it is plain, consists in a power to do, or not to do, to do, or to forbear doing, as we will. This cannot be denied. But this seeming to be all that is necessary to prove that the will is free, will not hold: for liberty, which is but a power, belongs only to agents, and cannot be an attribute or modification of the will, which is also but a power.”

Locke goes on to analyze the causes of human action. All action is motivated by uneasiness — the desire to remove some present pain or lack. The “greater good” does not always move us; it is the immediate uneasiness that determines the will. This uneasiness arises from the absence of some good, and the mind has the power to suspend the execution of any desire, to examine the options, and to choose the best course. This power of suspension is the foundation of human freedom: we are not determined by the strongest present desire but can deliberate and choose among competing goods.

The chapter then examines the various sources of uneasiness: bodily pains, desires for absent goods, passions, and the “desire of happiness” itself. Locke distinguishes between the desire for happiness (which is constant and universal) and the particular uneasiness that moves us to action at any given moment. The mind can change its evaluative judgments over time, leading to changes in desire and action. This analysis provides the psychological framework for Locke’s moral theory: moral responsibility requires that we have the power to suspend our desires, examine them, and choose according to reason.

This chapter is central to the entire Essay because it establishes the Lockean theory of action, freedom, and moral agency. The idea of power is the foundation for understanding causality, agency, and moral responsibility. Locke’s claim that liberty belongs to the agent, not the will, is a crucial philosophical move that avoids the pitfalls of both determinism and libertarian free will while preserving moral responsibility. The power of suspension is what makes us moral beings: we are not slaves to immediate desire but can deliberate and choose the good.

Book II — Chapter 22: Of Mixed Modes

In Chapter 22, Locke introduces the concept of mixed modes — complex ideas that combine simple ideas of different kinds and that are often not found together in nature.

Locke defines mixed modes as “complex ideas which are made up of several simple ideas of different kinds, combined together by the mind, and which are not necessarily connected in nature.” Examples include beauty (a combination of color, shape, and the pleasure they produce), theft (the taking of another’s property without consent, which combines ideas of ownership, taking, and permission), murder (the taking of a life with intention and without right), and gratitude (a combination of receiving a benefit and the disposition to acknowledge and return it). These mixed modes are the creation of the mind; they are not copies of any real essences in the world.

Locke emphasizes that mixed modes are arbitrary in the sense that the mind is free to combine simple ideas in any way it chooses. The mind can make a complex idea that has no counterpart in reality — such as the idea of a centaur (horse + man) or a golden mountain (mountain + gold). But many mixed modes are given names in language, and these named mixed modes become the basis for moral and political discourse. The names of mixed modes are “the materials of our moral and political knowledge” — words like “justice,” “right,” “obligation,” “drunkenness,” “lying” all name mixed modes that the mind has constructed from simpler ideas.

This chapter establishes that the mind is not merely passive in receiving simple ideas but active in constructing complex ones. Mixed modes are products of the understanding’s power to combine, abstract, and name. They are the building blocks of moral and political philosophy, because moral concepts are not innate essences but human constructions. This has profound implications: moral truths can still be certain (as in mathematics) because they are about relations between ideas that we have made ourselves, not about external realities that we can only observe imperfectly.

Book II — Chapter 23: Of Our Complex Ideas of Substances

Chapter 23 is a major treatment of the complex ideas of substances — the things we take to exist independently in the world.

Locke begins by noting that the mind observes that certain simple ideas constantly go together. We perceive that a certain color, shape, solidity, taste, and smell always appear together, and we suppose that there is some substratum or support in which these qualities inhere. This substratum is the idea of substance in general — “a supposition of we know not what support of those qualities that are capable of producing simple ideas in us.” The complex idea of a particular substance (e.g., a horse, a stone, gold) is a combination of simple ideas that we believe to co-exist in a single thing, together with the obscure idea of the substratum that supports them.

Locke then examines the distinction between material substances and spiritual substances. Material substances (bodies) are characterized by the primary qualities of solidity, extension, figure, and mobility, along with secondary qualities (color, taste, etc.). Spiritual substances (minds) are characterized by the powers of thinking, willing, understanding, and perceiving — all derived from reflection. In both cases, our idea of substance is obscure: we do not know what the substratum is in itself; we only know the qualities that inhere in it. Locke writes:

“The idea of substance is nothing but the supposed but unknown support of those qualities we find existing, which we imagine cannot subsist without something to support them. We call that support substantia, which, according to the true import of the word, is ‘standing under’ or ‘upholding’.”

Locke applies this analysis to the idea of God. Our complex idea of God is formed by taking the simple ideas we have from reflection (thinking, willing, power, knowledge, goodness) and extending them to infinity. We combine these ideas into a complex idea of an eternal, all-powerful, all-knowing, perfectly good being. This idea of God is not innate but constructed by the mind from simple ideas of reflection, magnified to infinity. Locke insists that this idea is adequate for our purposes — we can know that God exists and what God’s attributes are, even though our idea of God’s essence (the substratum of these attributes) remains obscure.

This chapter establishes Locke’s theory of substance as a solution to the problem of what underlies the qualities we perceive. We do not have a clear idea of substance itself — only a “supposed, unknown support.” But we can form complex ideas of particular substances by combining the simple ideas that we observe to co-exist. This theory applies equally to body and mind, to material and spiritual substances. The idea of God is a special case where we extend our ideas of mental qualities to infinity.

Book II — Chapter 24: Of Collective Ideas of Substances

In Chapter 24, Locke examines collective ideas of substances — complex ideas formed by putting together several individual substances into a single idea.

Locke observes that the mind can take many individual substances and consider them as one thing. Examples include an army (a collection of men), a flock (a collection of sheep), a fleet (a collection of ships), and the universe (the collection of all bodies). These collective ideas are complex ideas formed by the mind’s power of composition, just as it forms complex ideas of individual substances. The difference is that the parts of a collective idea are themselves whole substances, not just simple ideas. Locke writes:

“The ideas of substances have in themselves a union, and are truly individuals; but our ideas of them are as much a combination of simple ideas as any other complex ideas are. The mind can make a complex idea of many particular substances, considered together as one thing, as of an army, a legion, a company, a fleet, etc.”

Locke notes that collective ideas are particularly important in social and political life. The idea of a society or commonwealth is a collective idea of human beings united under laws or government. The idea of a family is a collective idea of parents and children. These collective ideas are not mere fictions — they refer to real collections of individuals — but the unity is mental, not natural. The mind arbitrarily decides which collections to treat as a single thing, and language provides names for these collective ideas.

This chapter extends the analysis of complex ideas to include collective substances. It emphasizes that the mind’s power of composition is not limited to simple ideas but can also combine whole substances into larger wholes. This is important for understanding social and political philosophy: the unity of a nation or an army is a mental construction, not a natural kind.

Book II — Chapter 25: Of Relation

In Chapter 25, Locke turns to the topic of relation — the way the mind compares one idea with another and perceives connections between them.

Locke begins by defining relation as “the comparing of one idea with another.” Whenever the mind considers two things together and notices how they stand with respect to each other, it forms an idea of relation. The relation is not a quality of either thing considered alone; it arises from the act of comparison. The correlates are the two ideas being compared, and the relatum is the relation that holds between them. For example, when we compare a father and a son, we form the relation of “fatherhood” on one side and “sonship” on the other. These relations are relative terms — they have no meaning apart from the correlation.

Locke distinguishes between natural relations and instituted relations. Natural relations are those based on some real connection in the world, such as cause and effect (where one thing produces another) or generation (where one thing gives rise to another). Instituted relations are those created by human convention, such as the relation of “husband and wife” (established by marriage) or “citizen and magistrate” (established by government). Both types of relation involve the mind comparing two ideas, but the basis of the comparison differs. Locke writes:

“The nature of relation consists in the referring or comparing two things one to another; from which comparison one or both comes to be denominated. And if either of those things be removed or cease to be, the relation ceases, and the denomination is lost, though the other remains.”

This chapter establishes that relation is a fundamental operation of the mind. We do not merely perceive simple ideas and complex ideas; we also perceive how ideas stand in relation to each other. Relations are not qualities of things but “extraneous denominations” — ways of considering things that depend on the mind’s act of comparison. This is important for epistemology because many of our most important ideas (cause, effect, identity, diversity) are relational ideas.

Book II — Chapter 26: Of Cause and Effect, and Other Relations

Chapter 26 examines the specific relation of cause and effect and several other important relations.

Locke defines cause as “that which produces any simple or complex idea,” and effect as “that which is produced.” The idea of causation arises from observing that one thing invariably precedes another and brings it into being. We observe fire producing heat, the sun producing light, a father producing a child (through generation), and a craftsman producing a table (through making). In each case, we notice a power in the cause to produce the effect, and we form the relation of cause and effect between them.

Locke then examines other important relations. Identity and diversity are relations of comparison: we determine whether a thing is the same as itself over time (identity) or different from other things (diversity). Proportion is the relation between quantities (more or less, greater or smaller). Natural relations include relations of time (before and after, sooner and later), place (above and below, near and far), and connection (cause and effect). Instituted relations include relations of property (owner and owned), authority (ruler and subject), and agreement (promisor and promisee).

This chapter shows that relation is a pervasive feature of human understanding. We do not merely have ideas of things; we have ideas of how things stand in relation to each other. The relation of cause and effect is particularly important because it underlies all reasoning about the physical world and all human action. Understanding causation requires understanding power — the active power in the cause to produce the effect — which connects this chapter back to the earlier analysis of power in Chapter 21.

Book II — Chapter 27: Of Identity and Diversity

Chapter 27 is one of the most influential in the entire Essay, containing Locke’s famous theory of personal identity.

Locke begins by stating that the ideas of identity and diversity arise from the mind’s power of comparison. We compare a thing with itself at different times to determine if it is the same thing. The principle of identity is simple: a thing is the same with itself and different from every other thing. But applying this principle requires determining what counts as “the same thing” for different kinds of entities. For inanimate bodies, identity is determined by the continuity of the same matter — a lump of gold remains the same lump as long as no atom is added or removed. For living organisms (plants, animals), identity is determined by the continuity of the same life — the same organization of parts that sustains the vital functions. A tree remains the same tree even though its matter is constantly changing, because the same life continues. For human beings, identity is determined by the continuity of the same body — the same organized living body that persists over time.

Then Locke turns to personal identity, which is the most important case. He defines a person as “a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing, in different times and places.” Personal identity is determined by consciousness — the ability to remember past thoughts and actions as one’s own. A person is the same person over time as long as consciousness connects past and present experiences. Locke writes:

“For, since consciousness always accompanies thinking, and it is that which makes every one to be what he calls self, and thereby distinguishes himself from all other thinking things, in this alone consists personal identity, i.e. the sameness of a rational being, as far as this consciousness can be extended backwards to any past action or thought.”

Locke draws dramatic conclusions from this analysis. If the same consciousness can be transferred to different substances (e.g., if the consciousness of a prince enters the body of a cobbler), the person would remain the same despite the change of body. Conversely, if a single substance loses consciousness of its past (as in amnesia or sleep), it is not the same person in the eyes of justice — we cannot punish someone for a crime they cannot remember committing. Locke applies this to the doctrine of the resurrection: personal identity after death depends on consciousness being reunited with the body, not on the identity of the material particles.

This chapter revolutionized the philosophy of personal identity. Locke’s argument that personal identity consists in consciousness, not in the substance of soul or body, had enormous implications for theology, ethics, and law. It provided a framework for understanding moral responsibility: we are accountable only for those actions that belong to our present consciousness. It also allowed for the possibility of a single substance undergoing multiple persons (as in cases of split personality) or multiple substances belonging to a single person (as in the prince and the cobbler).

Book II — Chapter 28: Of Other Relations

In Chapter 28, Locke examines several additional types of relations, particularly moral relations and proportional relations.

Locke begins by noting that relations can be divided into three types: natural relations (based on the constitution of things), instituted relations (based on human convention), and moral relations (based on the conformity or disagreement of human actions with a rule). Moral relations are crucial for ethics: they involve comparing an action with a law or rule to determine its moral quality. Locke identifies three kinds of laws or rules:

1. The divine law — the law of God, discovered through revelation or reason. This is the standard for determining sin and duty. 2. The civil law — the law of the state, enforced by the magistrate. This determines criminal and civil offenses. 3. The law of opinion or reputation — the law of fashion or social custom, enforced by praise and blame. This determines virtue and vice in the social sense.

Each of these laws provides a standard against which actions are measured. The moral relation is the agreement or disagreement of the action with the rule. For example, an action that agrees with the divine law is “right”; one that disagrees is “wrong.” An action that agrees with the civil law is “lawful”; one that disagrees is “criminal.” An action that agrees with the law of opinion is “virtuous”; one that disagrees is “vicious.” Locke writes:

“Good and evil, as has been shown, are nothing but pleasure and pain, or that which occasions or produces pleasure or pain in us. Moral good and evil, then, is the conformity or disagreement of our voluntary actions to some law, whereby good or evil is drawn on us from the will and power of the law-maker.”

This chapter establishes the relational nature of moral concepts. Moral terms like “good,” “evil,” “right,” and “wrong” do not name properties of actions themselves but relations between actions and laws. Moral knowledge is therefore relational knowledge: we know that an action is wrong if we know that it violates a law that we are obligated to obey. This connects Locke’s theory of relation to his broader moral philosophy.

Book II — Chapter 29: Of Clear and Distinct, Obscure and Confused Ideas

In Chapter 29, Locke examines the qualities of ideas — whether they are clear or obscure, distinct or confused.

Locke begins by borrowing terms from Descartes: an idea is clear when it is “present to the mind with all its original vividness and freshness” — like seeing an object in broad daylight. An idea is obscure when it is faint, dim, or indistinct — like seeing something through a fog. An idea is distinct when the mind perceives it as different from all other ideas — it has clear boundaries and can be separated from others. An idea is confused when it is not clearly distinguished from other ideas — the boundaries are blurred and the mind cannot tell where one idea ends and another begins.

Locke applies these distinctions to different kinds of ideas. Simple ideas are usually clear and distinct because they come directly from sensation or reflection. The idea of white is perfectly clear and distinct — we know exactly what it is and how it differs from black or red. Complex ideas can be clear or obscure depending on how well we understand their components. The idea of a triangle is clear and distinct because we can clearly perceive its three sides and three angles. The idea of God is clear but not distinct in the same way — we have a clear idea of God’s attributes (power, wisdom, goodness) but the nature of the divine substance is obscure to us.

This chapter provides a taxonomy for evaluating the quality of our ideas. Clear and distinct ideas are the foundation of knowledge — when our ideas are clear and distinct, we can reason reliably about them. Obscure and confused ideas lead to error and misunderstanding. Locke’s message is that we must strive to make our ideas as clear and distinct as possible, especially in matters of morality and religion where confused ideas cause the most harm.

Book II — Chapter 30: Of Real and Fantastical Ideas

Chapter 30 distinguishes between real ideas and fantastical ideas.

Locke defines real ideas as those that have a foundation in nature — they conform to the real existence of things or to the natural powers of the mind. Simple ideas are always real because they are produced in us by the actual operation of objects on our senses. The idea of red is real because it is caused by the power of an object to produce that sensation in us. Complex ideas of substances are real when they are combinations of simple ideas that actually co-exist in nature — the idea of gold is real if it corresponds to the actual qualities of the metal. Fantastical ideas are those that have no foundation in nature — they are mere inventions of the mind with no correspondence to reality. The idea of a centaur (half man, half horse) is fantastical because no such being exists.

Locke notes that the same complex idea can be real for one purpose and fantastical for another. The idea of a golden mountain is fantastical if taken as a substance (no such mountain exists) but real if taken as a mixed mode (the mind can legitimately combine the ideas of gold and mountain). This distinction is important for epistemology: we must be careful not to treat our own mental constructs as if they were real existences. The “real” is what exists independently of our thinking; the “fantastical” is what exists only in our imagination.

This chapter establishes a criterion for evaluating the content of our ideas. Real ideas are the foundation of knowledge about the world; fantastical ideas are the source of error when mistaken for realities. The distinction is particularly important for science and philosophy: we must ensure that our ideas of substances correspond to what actually exists, and we must not mistake our own conceptual inventions for objective realities.

Book II — Chapter 31: Of Adequate and Inadequate Ideas

In Chapter 31, Locke examines the distinction between adequate ideas and inadequate ideas.

Locke defines an adequate idea as one that “perfectly represents all the parts and properties of the thing it is supposed to represent.” An inadequate idea is one that represents only part of the thing. Locke argues that simple ideas are always adequate: the idea of white perfectly represents the power in the object to produce whiteness, because we have no other idea of whiteness than the sensation itself. Similarly, the ideas of modes (number, figure, space) are adequate because the mind itself creates these ideas and knows all their parts. The idea of a triangle is adequate because the mind has constructed it and knows its definition and properties.

But complex ideas of substances are always inadequate. Our idea of gold, for example, includes only some of the qualities of gold (yellowness, heaviness, malleability) but not all of them. There are many properties of gold—its chemical composition, its atomic structure, its hidden powers—that we do not know. Our idea of gold is therefore incomplete and inadequate. Locke writes:

“Our ideas of substances are, as has been shown, collections of simple ideas that are supposed to be united in one subject, and to co-exist in it. But as we cannot comprehend all the qualities that are in any substance, we never have an adequate idea of any substance.”

This chapter establishes a crucial limitation on human knowledge. We can have adequate ideas of simple ideas and modes because we either receive them directly or create them ourselves. But our ideas of substances are always partial and incomplete because substances have an infinite number of properties that we cannot fully grasp. This does not mean our ideas are useless—they are sufficient for practical purposes—but it means we can never have complete knowledge of the real essences of things.

Book II — Chapter 32: Of True and False Ideas

Chapter 32 examines whether ideas themselves can be called true or false.

Locke begins by arguing that ideas, considered in themselves, are never true or false. An idea of a centaur is not false—it is simply an idea. Truth and falsehood are properties of propositions — statements that affirm or deny something about ideas. When we say “There is a centaur in the garden,” we make a proposition that can be true or false. But the idea of a centaur itself is neither true nor false; it is what it is.

However, Locke allows that ideas can be called true or false in a secondary sense, when they are considered in relation to something outside themselves. An idea is “true” in this sense when it conforms to its archetype—when the idea of a substance corresponds to the actual thing, or when the idea of a mode corresponds to the definition the mind has established. An idea is “false” in this sense when it is taken to represent something that it does not actually represent—when we mistake an idea of a mode for an idea of a substance, or when we think an idea is adequate when it is not. Locke writes:

“Truth or falsehood, being an affirmation or negation of one idea concerning another, can never belong to ideas considered singly. It is a mistake to think that our ideas are, in themselves, true or false; they are only what they are.”

This chapter clarifies an important epistemological point. Ideas are not the bearers of truth; propositions are. The confusion arises because we often use ideas as the materials of propositions without realizing it. When we say “I have a true idea of gold,” we mean that the proposition “Gold has these qualities” is true. The distinction helps avoid confusion in philosophical reasoning.

Book II — Chapter 33: Of the Association of Ideas

The final chapter of Book II introduces the concept of the association of ideas — the way in which ideas become linked in the mind through custom and habit rather than through natural connection.

Locke notes that some ideas have a natural connection with each other—the idea of fire is naturally connected with the idea of heat, because fire produces heat. But the mind also creates unnatural connections between ideas that have no necessary relation. These associations are formed by chance, custom, education, or passion. For example, a child who is frightened by a spider may forever associate the idea of a spider with fear, even though spiders are not naturally frightening. Locke writes:

“Some of our ideas have a natural correspondence and connection one with another: it is the office and excellency of our reason to trace these, and hold them together in that union and correspondence which is founded in their peculiar beings. Besides this, there is another connection of ideas wholly owing to chance or custom; ideas that in themselves are not at all of kin, come to be so united in some men’s minds that it is very hard to separate them.”

Locke argues that these unnatural associations are the source of many errors, prejudices, and irrational behaviors. When ideas are wrongly associated, the mind cannot separate them even when reason demands it. This explains why people hold on to irrational beliefs despite evidence to the contrary—the ideas have become so strongly associated that they cannot be dissociated by reasoning. Locke suggests that this is the cause of many intellectual errors and moral failings.

This chapter is remarkable for introducing the concept of association into philosophy—a concept that would later be developed by David Hume and the British empiricists into a full theory of mental operations. Locke’s brief treatment here identifies a fundamental mechanism of mental life: ideas can become linked through experience and habit, creating patterns of thought that are not always rational. The association of ideas explains how customs, prejudices, and superstitious beliefs arise and persist, and it suggests that education and careful discipline are needed to form good mental habits.

Book III — Of Words: Chapter 1 — Of Words or Language in General

We are now in Book III, where Locke turns from ideas to words — the signs by which we communicate our ideas to others.

Locke begins by observing that God designed humans for society and therefore gave them the capacity for speech. Words are “sensible signs” used to represent ideas and communicate them to others. The primary use of words is to stand for ideas in the mind of the speaker. When I say “dog,” the word stands for my idea of a dog, and I intend it to raise a similar idea in the mind of my hearer. Words are therefore arbitrary — there is no natural connection between a word and the idea it stands for; the connection is established by human convention. Locke writes:

“Words, in their primary or immediate signification, stand for nothing but the ideas in the mind of him that uses them, how imperfectly soever or carelessly those ideas are collected from the things which they are supposed to represent.”

Locke notes that words are necessary for communication but also introduce significant difficulties. Because words stand for ideas that may be unclear, confused, or different from person to person, language is a source of misunderstanding and error. The goal of Book III is to examine the imperfections of language and to suggest remedies for the confusion they cause.

This chapter establishes the fundamental relationship between words and ideas that governs Locke’s philosophy of language. Words are signs, ideas are what they signify, and things are what the ideas represent. This three-part structure (words → ideas → things) will guide the analysis of language throughout Book III. The arbitrary nature of linguistic signs means that language is a human institution that can be reformed and improved.

Book III — Chapter 2: Of the Signification of Words

In Chapter 2, Locke examines how words come to signify ideas and the problems that arise from this signification.

Locke begins by reaffirming that words “stand for nothing but the ideas in the mind of him that uses them.” When a person speaks, the words they use are supposed to be signs of their own ideas. But there are several difficulties. First, a person may use words without having any clear idea attached to them — they may use terms they have heard but not understood. Second, a person may use words that are ambiguous — the same word may stand for different ideas at different times. Third, a person may use words that have no real meaning at all — such as the jargon of scholastic philosophy.

Locke then examines how words come to signify general ideas. Most words are general terms — they stand for general ideas (like “man,” “horse,” “beauty”) rather than for particular ideas (like “this man,” “that horse”). The process by which words come to signify general ideas is abstraction: the mind separates the common features of many particular experiences and forms a general idea, to which a general name is attached. For example, the word “man” stands for the general idea of a rational animal, abstracted from particular men. Locke writes:

“General and universal belong not to the real existence of things; but are the inventions and creatures of the understanding, made by it for its own use, and concern only signs, whether words or ideas.”

This chapter establishes that universals are not real entities existing in the world but are creations of the mind for the purpose of classification and communication. The problem of universals — whether general terms refer to real entities or mental constructions — is resolved by Locke in favor of nominalism: general terms are signs of abstract ideas, not of real essences. This has profound implications for science and philosophy: classification is a human activity, not a discovery of natural kinds.

Book III — Chapter 3: Of General Terms

Chapter 3 continues the analysis of general terms and their connection to abstract ideas.

Locke argues that all words are general except for proper names (which name particular individuals). General terms are necessary because it is impossible to have a distinct name for every particular thing — there are too many individuals. Moreover, general terms allow us to communicate about classes of things and to engage in scientific reasoning. The process of forming general terms involves abstraction: the mind observes several particular things (e.g., several men) and removes (abstracts) the differences — such as height, weight, age, color — and retains only the common features (rationality, animality). The result is a general idea (the idea of “man”) that is “made by the mind” and not found in nature.

Locke then introduces the distinction between real essence and nominal essence. The real essence of a thing is its internal constitution — the unknown real structure that gives rise to its observable qualities. The nominal essence is the abstract idea to which the name is attached — the collection of observable qualities that we use to define the species. For example, the nominal essence of gold is “yellow, malleable, heavy, fusible, fixed” — the qualities we use to identify gold. The real essence is the unknown internal structure of gold that causes these qualities. Locke argues that in practice, we classify things by their nominal essences, not their real essences, because real essences are unknown to us. He writes:

“The measure and boundary of each species, whereby it is constituted that particular sort, and distinguished from others, is that we call its essence, which is nothing but that abstract idea to which the name is annexed; so that every thing contained in that idea is essential to that species.”

This chapter is crucial for understanding Locke’s philosophy of science and classification. The real essence of things is unknown; what we call “essences” in ordinary language are really nominal essences — the abstract ideas we have constructed. This means that species boundaries are human creations, not natural divisions. Two things belong to the same species if they share the same nominal essence — the same set of observable qualities — regardless of their real internal constitution. This had revolutionary implications for natural philosophy: it broke with the Aristotelian tradition of real essences and opened the way for empirical classification.

Book III — Chapter 4: Of the Names of Simple Ideas

In Chapter 4, Locke examines the names of simple ideas, arguing that they have special properties compared to the names of complex ideas or substances.

Locke begins by noting that the names of simple ideas are least doubtful and most certain of all names. When we say “white” or “sweet” or “cold,” everyone who has had the sensation knows exactly what is meant. There is no room for dispute about the meaning of simple idea names, because the ideas themselves are clear and distinct. However, simple ideas have a peculiar property: their names cannot be defined. You cannot define “white” to someone who has never seen white; you can only point to an example. Simple ideas are the atoms of language — the basic building blocks that cannot be analyzed into simpler components.

Locke also notes that the names of simple ideas are incommunicable to anyone who lacks the corresponding sensation. A blind person cannot understand the word “red” as it is understood by a sighted person, because they lack the idea. This has epistemological implications: knowledge that depends on simple ideas can only be communicated to those who have had the relevant experiences. The names of simple ideas are also least subject to controversy — no one disputes whether white is white or whether cold is cold. The disagreements arise when we combine simple ideas into complex ones and give them names.

This chapter establishes that simple idea names are the foundation of language. They are the most certain, the least ambiguous, and the most universally understood. But they also have limits — they cannot be defined, and they depend on shared experience. Understanding the nature of simple idea names is essential for avoiding confusion in language.

Book III — Chapter 5: Of the Names of Mixed Modes and Relations

Chapter 5 examines the names of mixed modes and relations — the most complex and problematic names in language.

Locke begins by arguing that the names of mixed modes are arbitrary collections of simple ideas, united by the mind and given a name. Unlike the names of substances (which are supposed to correspond to real things), the names of mixed modes have no external standard — the mind is free to combine simple ideas in any way it pleases, and the name stands for that combination. Examples include “murder,” “gratitude,” “justice,” “sacrilege,” “drunkenness.” These names do not copy anything in nature; they are human inventions for moral and social discourse.

Locke notes that the names of mixed modes are particularly prone to ambiguity and confusion. Different people may attach different ideas to the same word, leading to endless disputes. For example, what counts as “murder” depends on what simple ideas are included in the complex idea — does it require malice? Premeditation? Unlawfulness? Different legal systems may define it differently. The remedy is to fix the definitions of mixed mode names by specifying precisely which simple ideas they contain. Locke writes:

“The names of mixed modes, therefore, being such voluntary collections of simple ideas which the mind puts together, without reference to any archetypes, may be perfectly and exactly defined; because, when we define them, we declare nothing but our own thoughts.”

This chapter establishes that mixed mode names are the most controllable and definable of all names, because they are entirely human creations. We can define “gratitude” exactly by listing the simple ideas it contains (receiving a benefit, acknowledging it, desiring to return it). But this also means that disputes about mixed modes are often disputes about definitions — not about facts. The remedy for confusion is clear definition and explicit agreement about the meaning of terms.

Book III — Chapter 6: Of the Names of Substances

In Chapter 6, Locke turns to the names of substances — the most important and most problematic names in language.

Locke begins by noting that the names of substances (like “gold,” “horse,” “man”) are supposed to stand for species of things — classes of individual substances that share certain qualities. The problem is that we do not know the real essences of substances; we only know their nominal essences — the collections of simple ideas we observe to co-exist. The names of substances therefore stand for these nominal essences, not for the real essences. This leads to uncertainty in the application of substance names: different observers may include different simple ideas in the nominal essence, leading to disputes about whether a particular thing belongs to a species.

Locke examines several consequences. First, the boundaries of species are vague — there is no precise line between one species and another. Second, the names of substances are imperfect — they do not capture the full reality of the things they name. Third, the names of substances are subject to change as we discover new qualities of things. For example, the nominal essence of gold might originally have included only “yellow, heavy, malleable,” but later discoveries added “dissolves in aqua regia” and “has a specific gravity of 19.3.” The nominal essence expands as knowledge grows.

Locke applies this analysis to the most contested substance name: “man.” What is the nominal essence of man? Is it “rational animal”? Or “body of a certain shape”? Or something else? Locke argues that the definition of man is a matter of human convention, not of natural reality. The question of whether a monster born without reason is a man depends on which nominal essence we adopt — not on some metaphysical reality. He writes:

“The sorting of things under names is the work of the understanding, and is determined by the ideas we have of them, not by the real internal constitution of things.”

This chapter is central to Locke’s critique of Aristotelian science. The traditional view held that species are fixed by real essences and that biological classification discovers these natural kinds. Locke argues that classification is a human activity, based on observable qualities, and that species boundaries are conventional. This had revolutionary implications for natural philosophy and biology.

Book III — Chapter 7: Of Particles

In Chapter 7, Locke examines particles — the small words that connect ideas and indicate relations between them.

Locke begins by noting that particles (such as “is,” “is not,” “and,” “but,” “if,” “because,” “therefore,” “yet”) are not names of ideas but signs of the connections or operations of the mind. When we say “if,” we indicate that the mind is considering a condition. When we say “but,” we indicate a contrast or exception. When we say “therefore,” we indicate a logical conclusion. Particles are the glue that holds language together — they express the structure of thought rather than its content.

Locke criticizes the neglect of particles in philosophy. Most philosophers focus on the names of substances and modes, but the real difficulty in reasoning often lies in the proper use of particles. A single particle can change the entire meaning of a sentence — “if A then B” is very different from “A and B.” The proper understanding of particles is essential for clear reasoning and communication. Locke writes:

“Particles are not words of idea but words of relation — they are the signs of the mind’s operations, and therefore the most difficult part of language to understand and to use properly.”

This chapter highlights the importance of logical structure in language. The meaning of a sentence is not just the sum of its substantive words; it depends crucially on how those words are connected by particles. Understanding particles is essential for understanding the operations of the mind — the connections it makes between ideas in reasoning.

Book III — Chapter 8: Of Abstract and Concrete Terms

Chapter 8 examines the distinction between abstract terms and concrete terms.

Locke defines abstract terms as those that stand for qualities or properties considered separately from any substance — such as “whiteness,” “heat,” “justice.” Concrete terms are those that stand for substances or for qualities as belonging to substances — such as “white,” “hot,” “just.” The distinction reflects two ways of considering the same reality: we can consider whiteness in the abstract (the quality itself) or we can consider a white thing (a substance with that quality).

Locke argues that abstract terms are necessary for reasoning and classification. Without them, we could not form general ideas or make universal statements. But abstract terms can also lead to confusion when they are mistaken for real entities — when we think that “whiteness” is a thing rather than a quality. The problem of abstract entities in philosophy — Platonic forms, real universals — arises from treating abstract terms as if they named real substances. Locke insists that abstract terms are names of ideas, not of things.

This chapter reinforces Locke’s nominalism: abstract terms are signs of abstract ideas, which are mental constructions. They have no corresponding reality in the world. The confusion between abstract and concrete terms is a source of philosophical error, particularly in the tradition of scholastic realism.

Book III — Chapter 9: Of the Imperfection of Words

In Chapter 9, Locke examines the imperfections of language and how they lead to misunderstanding and error.

Locke identifies several sources of imperfection in words. First, obscurity — words may stand for ideas that are not clear or distinct in the speaker’s mind. Second, ambiguity — the same word may stand for different ideas in different contexts. Third, instability — a word may change its meaning over time or from speaker to speaker. Fourth, impropriety — words are used in ways that depart from their established meaning. These imperfections affect all language but are particularly troublesome in philosophy, morality, and law.

The remedy for these imperfections is definition — making clear what ideas words stand for. For simple ideas, definition is impossible (you cannot define “white”), so we must rely on examples or direct sensation. For mixed modes and relations, definition is both possible and necessary — we must specify which simple ideas are included. For substances, definition is imperfect because we do not know the real essence, but we can clarify the nominal essence by listing the observable qualities that define the species. Locke writes:

“The chief end of language in communication is to be understood. Words are signs of ideas: and those signs are of no use unless they raise in the hearer’s mind the same ideas they stand for in the speaker’s mind.”

This chapter establishes that language is an imperfect instrument for knowledge. The imperfections of words are not incidental but inherent in the nature of language as a system of arbitrary signs. The remedy is constant attention to meaning, careful definition, and the willingness to clarify when misunderstandings arise.

Book III — Chapter 10: Of the Abuse of Words

Chapter 10 examines the abuse of words — the deliberate or careless misuse of language that corrupts understanding.

Locke identifies several forms of verbal abuse. First, the use of words without any clear ideas attached to them — the “jargon” of scholastic philosophy that sounds impressive but means nothing. Second, the use of words with unsteady meanings — shifting the meaning of a term in the middle of an argument to confuse or deceive. Third, the use of affectation in language — choosing obscure or artificial words instead of common ones to appear learned. Fourth, the use of words that are taken for things — treating abstract terms or nominal essences as if they were real entities. Fifth, the use of words to conceal ignorance — employing technical terms to cover up a lack of understanding.

Locke is particularly harsh on the scholastic tradition, which he accuses of inventing meaningless terms like “substantial forms,” “vegetative soul,” “horror vacui,” and “faculties” as substitutes for real explanations. These terms explain nothing; they merely give a name to our ignorance. The abuse of words is one of the greatest obstacles to knowledge, because it creates the illusion of understanding where none exists. He writes:

“The greatest part of the disputes in the world are merely verbal — they are about the meaning of words, not about the nature of things. And if men would tell their ideas clearly, or define their terms, the controversies would vanish.”

This chapter is a powerful critique of the abuse of language in philosophy and science. Locke’s message is that clear language is essential for clear thinking. The misuse of words is not a minor problem but a major obstacle to knowledge and communication. The remedy is to use words only when we have clear ideas, to define our terms carefully, and to avoid jargon and obscurity.

Book III — Chapter 11: Of the Remedies of the Foregoing Imperfections and Abuses

The final chapter of Book III proposes remedies for the imperfections and abuses of language.

Locke offers several practical remedies. First, never use a word without a clear idea attached to it. This is the most fundamental rule — every word should have a definite meaning in the mind of the speaker. Second, use words with steady meanings — do not change the meaning of a term in the course of an argument. Third, define your terms, especially for mixed modes and substances. Definition should specify the simple ideas that compose the complex idea. Fourth, use words in their common acceptation — do not invent new meanings without explanation. Fifth, be careful with analogy — do not use words metaphorically when literal meaning is required.

Locke also recommends that philosophers should, when necessary, declare the meanings of their terms explicitly, and that they should be willing to correct their language when misunderstandings arise. The goal is to make language a clear and reliable instrument for communication and knowledge. He writes:

“The remedy for the imperfections and abuses of words is to use them with caution, to define them when necessary, and to keep their meanings steady. This is the only way to avoid the controversies that arise from the abuse of language.”

This chapter concludes the book on words by offering practical advice for clear communication. The remedies are simple but require discipline: constant attention to meaning, careful definition, and the willingness to clarify. Locke’s message is that the cure for verbal confusion is not more technical jargon but greater clarity and simplicity in language.

Book IV — Of Knowledge and Opinion: Chapter 1 — Of Knowledge in General

We now enter Book IV, where Locke turns to the nature of knowledge itself — what it is, how we acquire it, and what its limits are.

Locke begins by defining knowledge as “the perception of the connexion and agreement, or disagreement and repugnancy, of any of our ideas.” Knowledge is not about things directly but about the relations between our ideas. We know something when we perceive that one idea agrees or disagrees with another. This definition has important consequences: knowledge is always about ideas, not about things-in-themselves; and knowledge is always relational — it involves comparing ideas and perceiving their connections.

Locke identifies four kinds of agreement or disagreement between ideas: identity (the idea is the same as itself), relation (two ideas stand in a relation such as cause and effect), co-existence (two ideas are necessarily connected, as with gold and its qualities), and real existence (an idea corresponds to something that actually exists). Each type of agreement yields a different kind of knowledge. Knowledge is therefore not a simple faculty but a complex operation of comparing ideas.

This chapter establishes the foundation for Locke’s epistemology. Knowledge is the perception of agreement between ideas — it is not intuition of external realities but a mental operation on the contents of the mind. This definition limits knowledge to what our ideas can represent and connect.

Book IV — Chapter 2: Of the Degrees of Knowledge

In Chapter 2, Locke distinguishes three degrees of knowledge: intuitive, demonstrative, and sensitive.

Intuitive knowledge is the highest degree — it occurs when the mind perceives the agreement or disagreement of two ideas immediately, without any intervening ideas. For example, we know intuitively that “white is not black” or that “a circle is not a triangle.” This knowledge is certain, irresistible, and self-evident. It is the foundation of all other knowledge.

Demonstrative knowledge occurs when the mind cannot perceive the agreement or disagreement immediately but must use a chain of reasoning, with each step being intuitive. Mathematics provides the clearest example: we prove that the angles of a triangle equal two right angles by a series of intuitive steps. Demonstrative knowledge is as certain as intuitive knowledge, but it requires effort and attention. The mind must keep the chain of reasoning clear and not lose sight of the connections.

Sensitive knowledge is the lowest degree — it is knowledge of the existence of particular external objects through sensation. When we see the sun, we know that the sun exists — not with the certainty of intuition or demonstration, but with a degree of probability sufficient for practical life. Sensitive knowledge does not provide certainty about the real essences of things, but it gives us reliable information about the existence of objects affecting our senses. Locke writes:

“The perception of the agreement or disagreement of our ideas is the whole of knowledge. All that we know, we know by intuition, demonstration, or sensation.”

This chapter establishes a hierarchy of knowledge. Intuitive knowledge is the most certain; demonstrative knowledge follows; sensitive knowledge is the least certain but still reliable for practical purposes. Each degree has its proper domain and its proper certainty.

Book IV — Chapter 3: Of the Extent of Human Knowledge

Chapter 3 examines the extent of human knowledge — what we can and cannot know.

Locke argues that human knowledge is limited in several ways. First, we have no knowledge of things beyond our ideas — we cannot know the real essences of substances. Second, we have no intuitive or demonstrative knowledge of the existence of other finite beings — we only have sensitive knowledge, which is limited to what our senses perceive. Third, we have no direct knowledge of the spiritual world — we cannot intuit the nature of angels or other minds. Fourth, our knowledge of the co-existence of qualities in substances is limited — we cannot deduce all the properties of gold from our idea of it.

Despite these limitations, Locke argues that we have sufficient knowledge for our practical needs. We know enough to live well, to pursue happiness, and to understand our moral obligations. The limits of knowledge are not a defect but a condition of our finite nature. We should not pretend to knowledge we do not have, nor despair at what we cannot know. The proper attitude is intellectual humility — acknowledging our limits while using our powers as best we can. He writes:

“Our knowledge is not infinite; it is proportioned to our state, and suited to our condition. We have knowledge enough for the concerns of this life, and for the way to a better. It becomes us to be content with what we have, and not to complain of our narrowness.”

This chapter is the heart of Locke’s epistemology. It establishes the limits of human knowledge while affirming its sufficiency for practical purposes. The message is both humbling and liberating: we cannot know everything, but we can know enough.

Book IV — Chapter 4: Of the Reality of Human Knowledge

In Chapter 4, Locke addresses the question of whether our knowledge is real — whether it corresponds to things as they actually are.

Locke argues that our knowledge is real insofar as our ideas agree with their archetypes. For simple ideas, this agreement is guaranteed: simple ideas are produced in us by the real operation of objects on our senses. The idea of red is real because it is caused by the power of an object to produce that sensation. For complex ideas of modes, reality is guaranteed by the mind’s own construction — the idea of a triangle is real because the mind has made it and knows all its properties. But for complex ideas of substances, reality is uncertain — we cannot be sure that our nominal essences correspond to real essences.

The reality of knowledge depends on the truth of our propositions. A proposition is true when it affirms or denies something about ideas that actually agree or disagree. For example, “Gold is yellow” is true if the idea of yellow is actually part of the nominal essence of gold. But the proposition “Gold is malleable” is true only if malleability actually co-exists with the other qualities of gold. Our knowledge of substances is real but imperfect — it captures some of the qualities but not all. Locke writes:

“Our knowledge is real only so far as there is a conformity between our ideas and the reality of things. But what is that reality? It is the power of things to produce ideas in us, and the agreement of our ideas with those archetypes.”

This chapter establishes that human knowledge, though limited, is real. We can know that our simple ideas correspond to real powers in objects, and we can know that our mode ideas correspond to our own constructions. The uncertainty lies only in our knowledge of substances, where our ideas may not fully capture the real essences.

Book IV — Chapter 5: Of Truth in General

Chapter 5 examines the nature of truth — what it is and how it relates to knowledge.

Locke defines truth as “the joining or separating of signs, as the things signified agree or disagree with one another.” Truth is primarily about propositions — statements that affirm or deny something. A proposition is true when it correctly represents the agreement or disagreement of ideas. For example, “All men are mortal” is true if the idea of man includes the idea of mortality (or if mortality necessarily follows from being human).

Locke distinguishes between mental truth (the agreement of ideas in the mind) and verbal truth (the agreement of words with ideas). Mental truth is the primary form; verbal truth is the expression of mental truth in language. Truth is also distinguished from probability: truth is certain knowledge; probability is opinion grounded in evidence but lacking certainty. Locke emphasizes that truth is the goal of all inquiry and that we should be diligent in pursuing it.

This chapter clarifies the relationship between knowledge and truth. Knowledge is the perception of agreement between ideas; truth is the propositional expression of that perception. The two are intimately connected: where there is knowledge, there is truth; but there can be truth in mere opinion (when we happen to state what is true without knowing it).

Book IV — Chapter 6: Of Universal Propositions

In Chapter 6, Locke examines universal propositions — statements that apply to all members of a class.

Locke argues that universal propositions are possible only where we have certain knowledge of the connection between ideas. In mathematics, we can make universal propositions (like “All triangles have three angles”) because our ideas of modes are perfectly clear and we know all their properties. In morality, we can make universal propositions (like “Where there is no property, there is no injustice”) because moral ideas are also modes whose properties we can deduce.

But in natural philosophy, universal propositions are almost impossible. We cannot say “All gold is malleable” with certainty because we do not know the real essence of gold — we cannot be sure that malleability necessarily follows from the internal constitution of gold. All universal propositions about substances are only probable, not certain. Locke writes:

“Universal propositions of whose truth or falsehood we can have certain knowledge, are not of the essence of things but of the relations of our ideas. The sciences of mathematics and morality are capable of demonstration; natural philosophy is not.”

This chapter establishes a crucial distinction between sciences that deal with modes (mathematics, morality) and sciences that deal with substances (natural philosophy). The former can achieve certainty; the latter can only achieve probability. This had profound implications for the status of physics and chemistry as sciences.

Book IV — Chapter 7: Of Maxims

Chapter 7 examines maxims — self-evident principles that are supposed to be the foundation of all knowledge.

Locke argues that maxims (like “What is, is” and “It is impossible for the same thing to be and not to be”) are not the foundation of all knowledge but are merely particular truths that the mind can perceive intuitively when it has the relevant ideas. These principles are discovered by the mind as it compares ideas, not innately impressed upon it. The mind does not need maxims to reason; it can perceive connections between ideas directly.

Locke is critical of those who treat maxims as the sole foundation of reasoning. He argues that this leads to a slavish dependence on formulas and a neglect of the direct perception of ideas. The proper method of inquiry is to examine ideas themselves, not to appeal to abstract principles. The role of maxims is merely to remind us of what we already know when our ideas are clear.

This chapter continues Locke’s critique of innate principles. Maxims are not the foundation of knowledge but only serve as aids for fixing assent once the agreement or disagreement of ideas has been directly perceived. Having disposed of the pretensions of maxims, Locke turns to the sort of propositions that, although certain, add nothing to real knowledge.

Book IV — Chapter 8: Of Trifling Propositions

Locke distinguishes real knowledge from “trifling” propositions. A proposition is trifling when it adds nothing to the understanding of the subject. Identical propositions, such as “What is, is” or “A soul is a soul,” are certain but empty. Likewise, when a part of a complex idea is affirmed of the whole idea, as when we say “Every man is an animal” while “animal” is already included in the idea of “man,” the proposition is only verbal. Such truths do not enlarge knowledge; they merely unpack the idea already present.

“We can know the Truth, and be certain of it, and yet not know anything beyond it.”

Real knowledge requires that the predicate be a distinct idea connected with the subject, not merely a repetition of it.

Book IV — Chapter 9: Of Our Knowledge of Existence

Locke treats knowledge of existence under three heads: intuitive knowledge of our own existence, demonstrative knowledge of the existence of God, and sensitive knowledge of other finite beings. The first is the most certain of all. We cannot doubt that we exist, because doubting is itself an act of an existing being.

“I think, I reason, I feel pleasure and pain; can any of these be more evident to me than my own existence?”

This intuitive certainty is the foundation on which all other knowledge of existence rests.

Book IV — Chapter 10: Of Our Knowledge of the Existence of a God

From our own certain existence, we can demonstrate that something has existed from eternity. Whatever begins to exist must have a cause; and since all that we observe is finite, dependent, and caused, there must be a first being that is eternal, powerful, and intelligent. Locke insists that this knowledge is demonstrable through reason, not stamped innately on the mind.

“The knowledge of our own being we have by intuition. The existence of a God, reason clearly makes known to us.”

All the perfections of God are conceived by extending the finite ideas of our own perfections beyond their limits.

Book IV — Chapter 11: Of Our Knowledge of the Existence of Other Things

We know the existence of other things only through sensation. The actual reception of ideas from external objects produces what Locke calls “sensitive knowledge.” This is less certain than intuition or demonstration, but it is sufficient to guide life. The senses are reliable enough to distinguish waking experience from dreams and to assure us of the real existence of external objects.

“The certainty of things existing in rerum natura when we have the testimony of our senses is as great as we can desire.”

The senses do not always produce certainty, but they are the only windows through which we see the world outside our minds.

Book IV — Chapter 12: Of the Improvement of Our Knowledge

Knowledge is improved not by relying on syllogisms or general maxims, but by discovering intermediate ideas that show the agreement or disagreement of the ideas compared. The mind must attend to clear and distinct ideas and trace their connections step by step. Memory, patience, and the direct inspection of ideas are the true instruments of learning.

“Knowledge is to be got by the application of the mind to things themselves, and by considering their ideas.”

Book IV — Chapter 13: Some Further Considerations Concerning Our Knowledge

Our knowledge is only of ideas and their relations, and it is real only where those ideas agree with the actual constitution of things. Locke warns that we must not expect certainty where our ideas are not clear or where their connections are not perceivable. The scope and limits of our knowledge are set by the presence or absence of these perceivable relations.

Book IV — Chapter 14: Of Judgment

Because certainty is not attainable in most matters, God has given us judgment to supply the defect. Judgment is the act of taking ideas together or separating them without perceiving their agreement or disagreement to be certain. It is a supposition that something is so, based on probability rather than demonstration.

“The faculty which God has given man to supply the want of clear and certain knowledge is judgment.”

Through judgment we direct the ordinary affairs of life.

Book IV — Chapter 15: Of Probability

Probability is the appearance of agreement or disagreement upon fallible proofs. Its two grounds are the conformity of a proposition with our own experience and the testimony of others. The degree of probability depends on the number and credibility of witnesses, the consistency of reports, and agreement with our own observations. Testimony may establish matters of fact, but it cannot overturn the certain testimony of our senses or the clear perception of our ideas.

“Probability is the appearance of agreement upon fallible proofs.”

Book IV — Chapter 16: Of the Degrees of Assent

Assent must always be proportioned to probability. Locke sets out degrees: full assurance when testimony is unanimous and agrees with experience; belief or opinion when the evidence is strong but not perfect; conjecture when the evidence inclines one way; and doubt or distrust when the evidence is evenly balanced. A reasonable mind adjusts its confidence to the strength of the grounds.

“The grounds of probability are the conformity of anything with our own knowledge, observation, and experience, and the testimony of others.”

Book IV — Chapter 17: Of Reason

Reason is the faculty of deducing unknown truths from principles already known, whether intuitively or by demonstration. It has two parts: the inference of agreement or disagreement, and the perception of degrees of probability. Locke denies that syllogism is the great instrument of reason; it only arranges arguments that are already discovered. Reason is the last judge and guide in all matters of truth.

“Reason must be our last Judge and Guide in every Thing.”

Book IV — Chapter 18: Of Faith and Reason, and Their Distinct Provinces

Faith is assent to a proposition upon the credit of a divine proposer, not upon the deductions of reason. But reason must determine whether a proposition truly comes from God. No alleged revelation can oblige our assent if it contradicts the clear and certain knowledge of our ideas. Faith, therefore, cannot destroy reason; it must be examined, judged, and received through reason.

“Reason is the rule and measure of the certainty of what is revealed.”

Book IV — Chapter 19: Of Enthusiasm

Enthusiasm mistakes the strong persuasion of a particular person for divine revelation. Locke condemns this as a dangerous ground of assent, because it sets aside reason and substitutes an unexamined inner impulse. The mere feeling of certainty proves nothing. A man must judge whether his persuasion is consistent with reason and with the known marks of revelation.

“He that will not give himself up to the conduct of reason, but under the pretence of revelation, acts as if he were inspired, is led by no certain rule.”

Book IV — Chapter 20: Of Wrong Assent, or Error

Error arises from four causes: the lack of proofs, the lack of ability to use them, the lack of will to use them, and the use of wrong measures of probability. Among these wrong measures are doubtful principles taken as certain, received hypotheses, and the influence of passion or interest. Men assent wrongly when they follow these instead of examining the evidence fairly.

“Men often receive their opinions upon trust, without ever examining the grounds of them.”

Book IV — Chapter 21: Of the Division of the Sciences

Locke concludes the Essay with a division of all human knowledge into three provinces. The first is the knowledge of things as they are in themselves, their relations, and their operations: natural science. The second is the knowledge of what a rational agent ought to do for the attainment of happiness: ethics or practice. The third is the knowledge of the ways whereby knowledge is acquired and communicated: the doctrine of signs, or logic.

“All that can fall within the compass of human understanding, being either, First, the nature of things, as they are in themselves, their relations, and their manner of operation; or, Secondly, that which man himself ought to do, as a rational and voluntary agent, for the attainment of any end, especially happiness; or, Thirdly, the ways and means whereby the knowledge of both the one and the other is attained and communicated; I think science may be divided properly into these three sorts.”