Locke’s Essay Concerning Human Understanding – John Locke

An Inquiry into the Origin, Certainty, and Extent of Human Knowledge

The Epistle to the Reader

We begin before the Essay proper, with Locke’s letter to his reader explaining the work’s origin. This is not a formal part of the treatise but a personal account of how the book came to be written and what the reader should expect.

Locke writes that five or six friends, meeting in his chamber and discussing “a subject very remote from this,” found themselves quickly at a standstill by “the difficulties that rose on every side.” It occurred to him that before they could inquire into such matters, they needed to examine “our own abilities, and see what objects our understandings were, or were not, fitted to deal with.” He proposed this as the first step, and the company agreed. Having set down some hasty thoughts on the subject for their next meeting, Locke found his notes growing; the work has since been “spun out” to its present length, written by fits and intervals over nearly twenty years.

Locke acknowledges that he has written for “the lover of truth,” not for the contentious disputant. He warns the reader not to expect “a perfect and finished piece” and admits that some parts may appear “dark and intricate.” He calls his labor “the diversion of some of my leisure hours” and states plainly that his purpose is to clear the ground a little: to remove “some of the rubbish that lies in the way to knowledge.” He explicitly names the scholastic philosophers and their disputations, the “learned gibberish” of the schools, as one great source of this rubbish. The reader who seeks clear and distinct ideas, who wants to examine the foundations of knowledge rather than merely defend received opinions, is the reader for whom this book is intended.

This letter matters because it establishes Locke’s empiricist starting point and his critical intention. He is not building a system from first principles but responding to a practical difficulty in conversation. The work is defensive in a constructive sense: it aims to uncover the limits of the understanding so that we do not waste effort on questions beyond its reach. The metaphor of “clearing the ground” is central to the entire project.

Book I: Of Innate Notions — Chapter 1: Introduction

We open the Essay proper. Locke announces his grand design: to inquire into “the original, certainty, and extent of human knowledge, together with the grounds and degrees of belief, opinion, and assent.” He will not meddle with the physical nature of the mind — the “physical consideration of the mind” is not his subject. His method is that of an under-labourer, clearing away the rubbish that hinders the advancement of real knowledge.

Locke states his central purpose in unequivocal terms. He proposes “to examine our own abilities, and see what objects our understandings were, or were not, fitted to deal with.” This is the enquiry that the conversation with his friends had suggested. His aim is to discover “the horizon” that sets the bounds between the enlightened and the dark parts of human understanding. He illustrates his point with a comparison: it is no more shameful for a man to admit ignorance of what lies beyond his capacity than it is for a day-laborer to admit he cannot explain the workings of a clock. The vice is not in limited understanding but in pretending to certainty where we have none, or in disputing about matters that surpass our faculties.

“If by this inquiry into the nature of the understanding, I can discover the powers thereof; how far they reach; to what things they are in any degree proportionate; and where they fail us, I suppose it may be of use to prevail with the busy mind of man to be more cautious in meddling with things exceeding its comprehension; to stop when it is at the utmost extent of its tether; and to sit down in quiet ignorance of those things which, upon examination, are found to be beyond the reach of our capacities.”

Locke clarifies that this is not a skeptical work. He does not aim to undermine knowledge but to secure it by establishing its proper boundaries. He distinguishes between knowledge and judgment or opinion: knowledge is the perception of the agreement or disagreement of ideas; judgment is the supposition of such agreement without certainty. The work will examine both. He warns against the “malice” of those who, finding some things unknowable, conclude that nothing can be known at all. The proper response is to examine our faculties and accept their limits.

This introduction is decisive for the whole Essay. Locke is setting the stage for a revolutionary project: he will derive all knowledge from experience, rejecting innate ideas, and then show exactly what we can and cannot know. The careful reader will note that Locke’s stance is neither dogmatic nor skeptical but critical and constructive.

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

We enter the negative argument of the Essay. Locke’s first task is to demolish the doctrine of innate ideas — the belief, held by many of his contemporaries, that certain principles, both speculative and practical, are imprinted on the human mind at birth. This chapter attacks the claim that there are innate speculative principles.

Locke begins with the most common candidate for such an innate principle: “Whatsoever is, is.” This proposition, the principle of identity, is widely agreed to be self-evident. But Locke asks: is it universally assented to? He argues that it is not. Children and idiots, he points out, have no notion of this principle. If a principle were innate, it would be present in all rational beings; yet these groups give no sign of having it. “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 principle is in the mind but not actually thought of is, for Locke, nonsense: the mind is nothing without its perceptions.

The defenders of innate ideas have an answer: they say that innate principles are present implicitly or potentially, that reason discovers them when the mind matures. Locke demolishes this reply. If “implicit presence” means that the mind can eventually come to know them, then all propositions whatever could be called innate. Reason does not discover innate principles; it discovers all truths. If reason were required to bring them to light, then they are no more innate than any other known truth. Furthermore, many truths are discovered before children ever learn the principle of identity. A child knows that the same thing cannot be both sweet and bitter long before it can formulate “Whatever is, is.”

“It is false that the mind receives all its ideas from sense and reflection, and that these are the only sources of our knowledge: for if any ideas are innate, then there is another source besides sense and reflection.”

Locke also examines the supposed innate practical principles. These are moral rules that, it is claimed, are universally assented to. Locke argues that there is no moral rule that commands universal assent. For example, “Justice, and keeping of contracts, is that which most men seem to agree in.” Yet even this is violated by whole societies: the practice of theft among some peoples, or the custom of exposing children, shows that no practical principle is universally admitted. What passes for universal consent is actually the result of education, custom, and the fear of punishment. If there were innate practical principles, they would produce innate conscience — a universal internal judge. But conscience varies with culture and teaching. “Men’s actions prove that they do not assent to these rules.”

Locke concludes Book I’s central argument: there are no innate principles, speculative or practical. The mind at birth is a white paper, void of all characters, without any ideas. How does it come to be furnished? That question leads directly into Book II. The importance of this negative argument cannot be overstated: it clears the way for Locke’s positive account of knowledge as derived entirely from experience. If we accept his arguments, the entire edifice of innate ideas collapses, and we must look elsewhere for the origin of our ideas.

Book I: Of Innate Notions — Chapter 3: Other Considerations Concerning Innate Principles

Locke continues his attack, now addressing further arguments that the proponents of innate ideas might offer. He considers the claim that certain “maxims” or axioms, such as the principle of non-contradiction, are innate because they are first in the mind. Locke responds by examining what “first” means.

He argues that if by “first” we mean “first known,” then the simplest ideas — such as those of pleasure and pain — are far more likely candidates than abstract maxims. A child feels pain from the first moment of life; it does not reason about identity. “A child knows not that three and four are equal to seven, till he comes to be able to count seven, and has got the name and idea of equality.” The abstract axioms are late acquisitions, built upon many simpler ideas. They are not the foundation of knowledge but are known only after much experience.

Locke also considers the argument that innate principles are necessary for the preservation of the individual or the species. He grants that some truths are very useful for self-preservation, but this does not make them innate. Birds build nests and spiders weave webs without any innate ideas of architecture; they act by natural instinct, not by reasoning from principles. If innate principles were necessary for preservation, they would be equally manifest in all times and places, but they are not. Men in different cultures have very different moral codes, yet they survive.

“There cannot any one moral rule be proposed whereof a man may not justly demand a reason: which would be perfectly ridiculous and absurd if they were innate, or so much as self-evident.”

Locke adds a further argument: if there were innate principles, they would have to be known as soon as the mind begins to operate. But the mind is not born with any ideas; it receives them gradually through the senses. The idea of God, for instance, is not universal; whole nations have been discovered who have no notion of a deity. And where the idea of God exists, it varies greatly. If it were innate, it would be uniform. Locke concedes that there may be a “natural disposition” in the mind to develop certain ideas when the senses present the proper materials — but this is a far cry from innate propositions stamped on the mind at birth.

This chapter solidifies Locke’s position. He has now covered the major arguments for innate ideas — universal consent, implicit presence, priority in knowledge, and necessity — and found each wanting. The way is now clear for his positive account. The reader should note that Locke is not merely being destructive: by showing what innate ideas are not, he defines what ideas must be — products of experience. Book II will supply that positive account.

Book II: Of Ideas — Chapter 1: Of Ideas in General, and Their Original

We move now from the critical demolition of innate principles into the constructive heart of the work. This chapter announces Locke’s positive account of the original of all our ideas — where they come from and how they enter the mind.

All ideas, Locke states plainly, come from experience. The mind is initially a blank slate — he uses the metaphor of “white paper, void of all characters” — before experience writes upon it. When we ask where all the materials of reason and knowledge come from, “To this I answer, in one word, From EXPERIENCE.” There is no other source. All our ideas are either sensation or reflection, and these two exhaust the entire supply.

“All ideas come from sensation or reflection. 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.”

Sensation is the first source. Our senses convey into the mind distinct perceptions of things — yellow, white, heat, cold, soft, hard, bitter, sweet — and these produce ideas of sensible qualities. This is the sensation operation: the external objects themselves, through the senses, furnish the mind with ideas of what is outside it. The second source is reflection: the mind’s perception of its own internal operations. When the mind turns inward upon itself and observes its own activities — thinking, doubting, believing, reasoning, knowing, willing — these produce ideas of mental operations. Reflection is “the perception of the operations of our own mind within us, as it is employed about the ideas it has got.”

Locke insists these two are the only fountains of all ideas. Every idea that enters the understanding comes either through the senses from external objects or through the mind’s observation of its own internal processes. Children, he notes, acquire ideas gradually — first through sensation (nourishment, warmth, light), and only later through reflection as they begin to notice their own thinking. The mind does not have ideas before it receives them through these channels; it is originally empty, a tabula rasa.

This chapter matters because it establishes Locke’s fundamental principle for the entire Essay. Everything that follows — all the complex ideas, the distinctions between primary and secondary qualities, the investigation of knowledge itself — rests on this single claim: experience is the origin of all thought. The metaphor of white paper is not decorative; it is a deliberate rejection of the innatist picture. Where the innatists claimed the mind comes pre-stamped with principles, Locke asserts that the mind is passive in receiving its first materials, active only in combining them afterward.

Book II: Of Ideas — Chapter 2: Of Simple Ideas

Having established that all ideas come from sensation or reflection, Locke now examines the most basic units of thought: simple ideas. These are the atoms of the mental world — “the materials of all our knowledge” — and they cannot be further broken down or decomposed.

Simple ideas come into the mind “pure and unmixed,” without any composition. When a child sees the color red or feels the warmth of a fire, the mind receives a simple idea that is “one uniform appearance” and “not distinguishable into different ideas.” These simple ideas are the mind’s raw materials, and the mind is entirely passive in receiving them — “the understanding is merely passive” in the reception of simple ideas, able to accept them but not to create or destroy them.

“These simple ideas, when offered to the mind, the understanding can no more refuse to have, nor alter when they are there, than a mirror can refuse, alter, or obliterate the images or ideas which the objects set before it do therein produce.”

Locke distinguishes between simple ideas coming from one sense only, from more than one sense, from reflection only, and from both sensation and reflection. Those from a single sense include colors (sight), sounds (hearing), tastes (taste), smells (smell), and tangible qualities like heat and solidity (touch). Those from more than one sense include space, extension, figure, rest, and motion — these are perceived by both sight and touch. Ideas of reflection alone include perception and willing. Ideas from both sensation and reflection include pleasure, pain, power, existence, and unity.

The critical point here is that simple ideas are the foundation. The mind can combine them into complex ideas, compare them, and abstract from them — but it cannot invent a genuinely new simple idea. A blind man cannot have the simple idea of colors; a deaf man cannot have the simple idea of sounds. The mind’s constructive powers only work upon the materials delivered by sensation and reflection.

This chapter provides the building blocks for Locke’s entire theory of mind. By insisting on the passivity of the understanding in receiving simple ideas, Locke ensures that all knowledge is ultimately grounded in experience. The mind cannot fabricate its contents from nothing; it must receive them from the world. This is the epistemological foundation for empiricism: all ideas, however complex or abstract, are traceable back to simple ideas received through sensation or reflection.

Book II: Of Ideas — Chapter 3: Of Ideas of One Sense

Locke now provides a detailed inventory of the simple ideas delivered by individual senses. He begins with those that come through one sense only — ideas that have no entrance into the mind except through a particular sensory channel. Each sense has its own domain of ideas that cannot be communicated to or received by another sense.

The sense of sight delivers ideas of light and colors — “white, red, yellow, blue, with their several degrees or shades and mixtures, as green, scarlet, purple, sea-green, and the rest.” These are strictly visual; a blind man cannot have them. The sense of hearing delivers sounds — “tunes and all modulations of sound.” The sense of smell delivers “the smells and odors of things.” The sense of taste delivers “savors and relishes” — sweet, bitter, sour, salty. The sense of touch delivers “sensible qualities” that are tangible — heat, cold, solidity, hardness, softness, smoothness, roughness.

“There be some ideas which have admittance only through one sense, which is peculiarly adapted to receive them. Thus light and colors, as white, red, yellow, blue, with their several degrees or shades and mixtures, as green, scarlet, purple, sea-green, and the rest, come in only by the eyes.”

Locke emphasizes that these sensory ideas are incommunicable. “You cannot make a man understand a color by discourse,” he observes, if he has never seen it. A blind man who hears that scarlet is like the sound of a trumpet gains no genuine idea of scarlet. The only way to acquire such ideas is to receive them directly through the appropriate sense.

The chapter also notes that some senses, especially touch, deliver multiple distinct ideas — heat and cold are not the same as solidity, though both come through the sense of touch. Locke distinguishes solidity as particularly important, announcing that he will treat it at length in the following chapter. The other tangible qualities — hardness, softness, roughness, smoothness — are modifications of the idea of solidity or derive from it.

This inventory serves a crucial function in Locke’s system. By cataloging the simple ideas that enter through each sense, he provides a check against both rationalist excess and skeptical doubt. The catalog shows that the mind’s contents are limited to what the senses deliver; no idea is admissible that cannot be traced to this origin. At the same time, the incommunicability of sensory ideas explains why philosophy can go only so far in teaching about the world — some knowledge requires direct experience.

Book II: Of Ideas — Chapter 4: Of Solidity

Locke devotes an entire chapter to solidity because it is “the idea most intimately connected with, and essential to, body.” He considers it the most important simple idea contributed by touch, the one that distinguishes body from space and that underlies all our notions of physical substance.

Solidity is not the same as hardness. Hardness is the resistance of a body to change of shape or separation of parts; a diamond is hard. But solidity is the filling of space — the resistance that a body offers to the entry of another body into its own place. “This is the idea of body,” Locke writes, “by which we conceive it to fill space.” A soft body, like wool, has solidity no less than a diamond: both resist the approach of other bodies, though wool yields more easily. The difference between solidity and hardness is that solidity is absolute and universal to all bodies, while hardness is a matter of degree.

“The idea of solidity we receive by our touch; and it arises from the resistance which we find in body to the entrance of any other body into the place it possesses, till it has left it.”

Locke makes a critical distinction between solidity and pure space. Space is penetrable and yields to body without resistance; solidity is impenetrable. When I press my hand against a wall, the wall resists because it is solid — another body cannot occupy its place. But empty space offers no such resistance. This distinction is essential because Locke will later argue that space and body are different substances, and that the idea of solidity is the key difference between them.

The chapter also argues that solidity is “the foundation of all other ideas of body” — hardness, softness, roughness, smoothness, and even motion presuppose solid bodies. Without solidity, bodies could not push, resist, or interact. Solidity is thus “the most primary and most obvious” of the simple ideas belonging to body.

This chapter matters because it establishes one of the central distinctions in Locke’s metaphysics — the separation of body from space. The idea of solidity gives us our concept of material substance. Moreover, by showing that solidity is a simple idea received through touch, Locke reinforces his empiricist claim that all ideas, even the most fundamental, come from experience. There is no innate idea of substance or body; we acquire it through the resistance we feel when we press against the world.

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

Having treated ideas that come through one sense, Locke now considers simple ideas that reach the mind through more than one sense. These are few in number but philosophically significant. The ideas that come through both sight and touch are: space, extension, figure, rest, and motion.

Locke explains that these ideas are “suggested to the mind by both the eyes and the touch.” When we see a sphere and feel a sphere, we get the same idea of figure from both senses. When we see a moving object and feel an object moving across our hand, we get the same idea of motion. The same applies to extension — the perception of distance or magnitude — and rest.

“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; and we can receive and convey into the mind the ideas of the extension, figure, motion, and rest of bodies, both by seeing and feeling.”

Locke does not dwell on this chapter at length — it is a brief classification. But its importance lies in what it reveals about Locke’s empiricist method. He is systematically cataloging how every idea enters the mind, tracing each to its sensory origin. The fact that extension, figure, motion, and rest are delivered by two senses suggests that they are particularly fundamental — they are not accidental qualities of particular senses but structural features of how we perceive the world.

This classification also anticipates Locke’s later distinction between primary and secondary qualities. The ideas that come through multiple senses — extension, figure, motion, rest — will turn out to be ideas of primary qualities (qualities that really exist in bodies). Those that come through only one sense — colors, sounds, tastes — will be secondary qualities (powers in objects to produce sensations in us). The groundwork for that crucial distinction is laid here.

Book II: Of Ideas — Chapter 6: Of Simple Ideas of Reflection

Locke now turns to the other fountain of ideas: reflection. These are ideas that come from the mind’s observation of its own internal operations. The mind, having received simple ideas through sensation, can then turn its attention inward and observe what it does with those ideas. This inward observation yields a distinct set of simple ideas.

The two principal operations of the mind that reflection reveals are perception (or thinking) and volition (or willing). Perception is “the first simple idea of reflection” and “the first operation of all our intellectual faculties.” When the mind perceives something — either an external object through sensation or an internal operation through reflection — it has the idea of perception. Volition is the power to begin or forbear, continue or end, certain actions of the mind or body.

“The perception or thinking, and the volition or willing, are the two great and principal actions of the mind, which are the most frequently considered, and which are so constantly occurring that whoever will take the pains to observe what passes in his own understanding cannot but have them as clear and distinct ideas as any he has.”

Locke notes that these ideas of reflection come later in development than ideas of sensation. Children perceive the world around them before they reflect on their own perceiving. But once reflection begins, it produces ideas just as real and distinct as those from sensation. The mind’s observation of its own operations is “as clear and distinct” as any idea from the five senses.

This chapter establishes the second track of Locke’s empiricism. Experience includes not only external sensation but internal reflection. Rationalist critics sometimes accused Locke of reducing all knowledge to sense perception; this chapter shows the charge is false. Reflection is an independent source of ideas, and it gives us our concepts of thinking, willing, doubting, reasoning, and knowing — the very concepts we need to understand the mind itself. The empiricism of the Essay is therefore broader than mere sensationalism; it is an empiricism of inner as well as outer experience.

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

Locke now identifies simple ideas that come to the mind from both sensation and reflection. These are ideas that arise from the operation of external objects on the senses and also from the mind’s observation of its own internal operations. They are, so to speak, universal categories of experience that apply to both the outer and inner worlds.

The ideas that come from both sources are: pleasure, pain, power, existence, unity, succession, and duration. Pleasure and pain, Locke explains, are present in “almost all that we think or feel.” We receive pleasure from agreeable sensations and from satisfying mental operations; we feel pain from disagreeable sensations and from mental distress. These ideas are “annexed” to almost every idea we have, making them extremely common.

“There be some simple ideas which we receive both from sensation and reflection, as pleasure, pain, power, existence, unity, succession, and duration. For these are ideas that we have of things without us, by experiencing them in ourselves; and they are also suggested to the mind by every external body that produces any thought in us.”

Power is a particularly important idea. From sensation, we observe that one body can move another, producing change — this gives us the idea of power. From reflection, we observe that our own minds can will and move our bodies — this also gives us the idea of power. Existence is suggested by every object we perceive (sensation) and by every thought we have (reflection). Unity is suggested by every distinct thing, whether a body or an idea. Succession comes from the observation of one thought following another in the mind and from the movement of external bodies.

Locke notes that these ideas are so pervasive that we rarely notice them as distinct simple ideas. They are “the modification of other simple ideas” and are “so constantly annexed to all our other ideas” that we take them for granted. Yet they are genuine simple ideas that enter the mind through both channels of experience.

This chapter is crucial because it provides the basis for many of Locke’s most abstract concepts. The ideas of power, existence, unity, and succession will be built upon later to form complex ideas of substance, cause and effect, number, and time. By showing that these fundamental categories come from experience, Locke completes his inventory of the raw materials from which all knowledge is constructed. There is no idea in the understanding that cannot be traced to one of these two fountains — sensation providing ideas of external objects, reflection providing ideas of internal operations, and both together providing the most general categories of thought.

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

We are now at a critical juncture in Book II where Locke pauses the inventory of simple ideas to introduce a distinction that will underpin his entire theory of perception and the qualities of bodies.

Locke begins by noting that our simple ideas are not arbitrary fictions but are produced in the mind by the operation of external objects on our senses. This raises the question of what exactly it is in bodies that produces these ideas in us. To answer this, Locke distinguishes between what he calls “Ideas” in the mind and the “Qualities” in bodies that produce them. A quality is defined as the power in any object to produce an idea in the mind. So when we speak of a snowball having the ideas of whiteness, coldness, and roundness, what we really mean is that there are powers in the snowball to produce those ideas in us.

Locke then divides these qualities into two sorts. Primary qualities are those that are “utterly inseparable from the body, in what state soever it be.” These include solidity, extension, figure, motion or rest, and number. No matter how much a grain of wheat is divided, it still retains these qualities; each particle has its own solidity, extension, figure, and mobility. The mind perceives these qualities as actually existing in the bodies themselves, and the ideas of primary qualities are resemblances of those qualities — the pattern in the object is actually like the idea in the mind.

“The Ideas of primary Qualities of Bodies, are Resemblances of them, and their Patterns do really exist in the Bodies themselves; but the Ideas, produced in us by these Secondary Qualities, have no resemblance of them at all.”

Secondary qualities, by contrast, are not really in the objects themselves. They are powers in objects to produce sensations in us — such as colors, sounds, tastes, smells, and heat or cold. These qualities are nothing in the objects themselves but the power to produce these sensations in us by means of the primary qualities (specifically, the bulk, figure, texture, and motion of the insensible parts). The idea of heat or whiteness in the mind has no more resemblance to anything in the body than the pain caused by a knife resembles the sharpness of its edge. The light, the color, the sound — these are not in the object but are effects produced in the perceiver.

Locke then introduces a third category: “powers” that are not simple ideas but are relations. These are powers in objects to produce changes in other objects — such as the power of fire to melt wax or the power of the sun to make wax white. These are not simple ideas of sensation but are considered as relations between bodies. The distinction matters because what we normally call “qualities” in objects are actually a mix of these different types: the primary qualities that really are in the body, the secondary qualities that are powers to produce sensations, and the tertiary powers to produce changes in other bodies.

This chapter is foundational for Locke’s epistemology because it establishes that our perceptual experience is not a direct mirror of reality. We have direct access only to our ideas; the primary qualities give us knowledge of how things really are in the world, but secondary qualities are merely subjective effects. This distinction will become crucial later when Locke discusses the nature of substance, the limits of human knowledge, and the distinction between real and nominal essences. It also provides a framework for understanding how the same physical object can appear differently to different perceivers — the primary qualities remain constant, while the secondary qualities vary.

Book II: Of Ideas — Chapter 9: Of Perception

We move now to the first of the simple ideas of reflection, beginning with perception, which Locke identifies as the first and simplest operation of the mind.

Perception is defined as “the first Operation of all our intellectual Faculties, and the inlet of all Knowledge into our Minds.” It is the act of the mind (or, as Locke carefully notes, “the impression made on the Mind by the Operation of external Bodies”) that gives us our simple ideas. Perception is not merely sensation, but the noticing or awareness of what the senses deliver. Locke emphasizes that the mind is passive in perception — it cannot refuse to have certain ideas when the senses are properly stimulated, just as a mirror cannot refuse to receive the images of objects placed before it.

Locke then makes a crucial observation: perception is not automatic even when sensation occurs. He notes that the mind often receives ideas without being consciously aware of them. For example, a person deep in thought may have sounds striking their ears but not perceive them. The mind can be so focused on one object that it does not notice others. This leads Locke to distinguish between sensation (the physical impression on the sense organs) and perception (the mind’s awareness of that impression). Perception requires an act of attention or notice — the mind must turn toward the idea.

“Perception, as it is the first Operation of all our intellectual Faculties, and the inlet of all Knowledge into our Minds, is, I think, the first and simplest Idea we have from Reflection.”

Locke then discusses the relation between perception and judgment. He argues that many things we think we perceive directly are actually the result of judgment. For instance, when we see a round globe of uniform color, what we actually perceive is a flat circle of varying light and shade; it is judgment (learned from experience) that interprets this as a sphere. The same occurs with distance and size: we learn to judge these from experience, not from direct sensation. This is why a child does not at first know that a distant object is far away — they have not yet learned to associate the smaller visual appearance with greater distance.

This chapter matters because it establishes perception as an active, cognitive process, not merely passive reception. The mind is not a blank slate that simply receives impressions; it must attend to them, notice them, and interpret them. This prepares the ground for Locke’s later account of memory, attention, and the active powers of the understanding. It also introduces the important idea that much of what we take to be direct perception is actually the product of rapid, unconscious judgment — an insight that will influence later discussions of the limits and reliability of human knowledge.

Book II: Of Ideas — Chapter 10: Of Retention

The next simple idea of reflection is retention, which is the power of the mind to keep ideas that it has received. Locke divides retention into two distinct operations.

The first is contemplation, which is keeping an idea actually in view for some time. When the mind perceives an object and continues to hold that idea before it after the object is removed, this is contemplation. The second and more important is memory, which is the power to revive ideas that have been in the mind but have been laid aside or out of sight. Memory is what makes the storehouse of our ideas available to us for future use. Without memory, all our knowledge would be limited to the present moment, and we could not reason, reflect, or learn from experience.

Locke notes that memory is essential to all intellectual life. If we had no memory, we would be perpetual infants, unable to connect one moment to the next. But memory is also imperfect: ideas fade over time and become more obscure. Some ideas we can recall vividly, others only dimly. Some are lost entirely. Locke compares this to the fading of colors in a cloth over time — the pattern remains but the vividness diminishes. This has important consequences for knowledge: our most certain knowledge is of present ideas; past knowledge depends on the reliability of memory.

“Memory, in the degree in which we have it, is the only Power of retaining our Knowledge, and the Foundation of all our Reasoning and Contemplation.”

Locke also observes that the mind often has ideas it cannot recall at will — they seem lost but later return unbidden. Animals also possess memory, which is evident from their ability to recognize places and persons and to learn from past experience. But Locke notes that the perfection of memory varies greatly among individuals. Some people have quick recall but poor retention; others have slow recall but retain things for decades. There is also the phenomenon of forgetting, where ideas that were once present are entirely lost, leaving no trace. The only cure for this is care and repetition — ideas that are frequently revived are retained longest.

This chapter is important because it establishes memory as a fundamental power of the understanding, not merely a passive storage but an active faculty that makes reasoning and knowledge possible. Locke will later rely on this account when discussing personal identity, the continuity of consciousness, and the basis of moral responsibility. Memory is what connects our past to our present, making us the same persons over time. Without it, there would be no self, no knowledge, and no basis for accountability.

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

This chapter introduces the mind’s active powers of discrimination and comparison, moving beyond the passive reception of ideas to their active manipulation.

Discerning is the power of the mind to distinguish between different ideas, to perceive that one idea is not another. This is the foundation of all clear and distinct knowledge. Locke argues that the ability to discern is what makes it possible for us to have distinct ideas at all. Without this power, all ideas would blend together in a confused mass. Discerning is also the basis of judgment: to judge that something is true or false is to perceive the agreement or disagreement between ideas.

The most important operation of discerning is wit and judgment. Wit lies in the quick and lively assembly of ideas that have a resemblance or congruity, which gives pleasure and delight. But this is not the same as judgment, which lies in the careful separation of ideas where the least difference is found. Locke famously states: “Wit lies in the assemblage of Ideas, and putting those together with quickness and variety, wherein can be found any resemblance or congruity… Judgment, on the contrary, lies quite on the other side, in separating carefully, one from another, Ideas, wherein can be found the least difference.” Wit creates poetry and metaphor; judgment creates science and truth.

“The difference between Wit and Judgment, is the difference between the quick and lively Assemblage of Ideas, and the careful Separation of them.”

Locke then discusses other operations of the mind. Comparing is the perception of the relations between ideas — whether they are equal or unequal, same or different, cause and effect. Compounding is the joining of simple ideas into complex ones, which is the basis of all general ideas and abstract notions. Naming is the attachment of words to ideas, which allows us to communicate them to others. Abstraction is the power to separate particular ideas from their circumstances and treat them as general representations, forming general ideas that stand for many particular things. This is the power that makes language and rational thought possible.

This chapter is crucial because it introduces the active powers of the understanding that will be essential for all higher cognitive operations. While simple ideas come passively through sensation and reflection, the power to compare, compound, and abstract is what allows the mind to build the complex structures of knowledge. Locke will rely heavily on abstraction in his theory of language, his account of general terms and essences, and his critique of innate ideas. The distinction between wit and judgment also shows Locke’s commitment to clear, distinct, precise thinking — the kind that distinguishes true knowledge from mere fancy.

Book II: Of Ideas — Chapter 12: Of Complex Ideas

We now come to a synthetic chapter where Locke defines complex ideas and classifies them into their major types. Up to this point, the discussion has focused on simple ideas; now the mind begins to build.

Complex ideas are those that the mind makes by combining several simple ideas together. The mind has three operations in this process: (1) combining several simple ideas into one compound idea, (2) bringing two ideas together and comparing them to perceive their relations, and (3) separating ideas from all other ideas that accompany them in reality — this is abstraction. Complex ideas thus arise entirely from the mind’s own activity, but the materials are always the simple ideas received from sensation and reflection.

Locke divides all complex ideas into three categories: modes, substances, and relations. Modes are complex ideas that do not contain the supposition of subsisting by themselves — they are dependent on substances. Examples include triangle, gratitude, murder, and a dozen. These can be further divided into simple modes (variations of one simple idea, like a dozen being a number of units) and mixed modes (combinations of several different simple ideas, like beauty being a combination of color, shape, and delight).

“The Mind being, as I have declared, furnished with a great number of the simple Ideas, conveyed in by the Senses, as they are found in exterior things, or by Reflection on its own Operations, takes notice also, that a certain number of these simple Ideas go constantly together; which being presumed to belong to one thing, and Words being suited to common apprehensions, and made use of for quick dispatch, are called, so united in one subject, by one name.”

Substances are complex ideas that are taken to represent distinct particular things subsisting by themselves. The idea of a substance is made by taking a collection of simple ideas that are observed to go constantly together (like a certain color, weight, hardness, and malleability that we call gold) and supposing some unknown substratum in which they inhere. The general idea of substance in general is the supposition of an unknown support for qualities that we find existing. Relations are complex ideas that arise from comparing one idea with another, such as cause and effect, father and son, bigger and smaller.

This chapter is the organizational key to much of the rest of Book II. Locke will spend many subsequent chapters exploring each category in detail — simple modes of space, duration, number, and power; the complex ideas of substances both material and spiritual; and the various relations such as cause and effect, identity, and moral relations. The framework established here shows how the mind, starting from the simplest elements of experience, actively constructs the entire edifice of human thought and knowledge.

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

Locke now begins his detailed examination of simple modes, starting with those that modify the idea of space. This chapter is the first in a series that will apply the framework of simple modes to specific fundamental ideas.

The author opens by defining a simple mode as a variation or combination of one simple idea, without the mixture of any other. The simplest examples come from space. The idea of space itself, Locke explains, is conveyed to the mind through the senses of sight and touch. Every visible or tangible object gives us the idea of extension, the partes extra partes quality of bodies — that is, the quality of having parts existing outside of other parts. From this, the mind acquires the idea of space in general, abstracted from any particular body.

From this simple idea, the mind generates various simple modes. The primary simple modes of space are distance, capacity, and figure. Distance is the idea of length between two points, whether considered as a straight line or otherwise. Capacity concerns the volume or three-dimensional extent of space. Figure arises from the termination of space in certain boundaries, producing ideas of shape — circle, square, triangle, and the infinite variety of geometrical figures. Each of these, Locke insists, is merely a modification of the single simple idea of extension.

“The infinite variety of figures that have nothing to do with bodies make up the infinite number of those simple modes of space.”

Locke then turns to the idea of place. He defines place as the relative position of a body with respect to other bodies. When we say something is “in a place,” we mean its distance and direction from some fixed reference point. The universe itself, considered as a whole, is in no place because there is nothing external to it by which its position could be determined. Place is thus a relational idea derived from space, not a distinct simple idea.

This matters because Locke is laying the foundation for his later discussions of infinity, duration, and number. By showing that space and its modes are all derived from sensation, he continues to prove that no idea is innate — even the most abstract geometrical concepts are modifications of sensory experience.

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

Locke turns from space to time, examining the idea of duration and its modes. This chapter parallels the previous one, showing how the mind constructs temporal ideas from simple experience.

The idea of duration, Locke argues, arises from reflection on the succession of ideas in our own minds. As we observe one thought following another, we acquire the idea of succession. When we consider the distance between any two parts of this succession, we obtain the idea of duration. This is not derived from motion, though motion often accompanies it; rather, motion is observed only through the succession of ideas in the mind. A man asleep, having no succession of ideas, has no perception of duration.

From duration, the mind generates the simple modes of time, moments, hours, days, years, and all the measures of temporal passage. These are all modifications of the simple idea of duration. Locke emphasizes that we measure duration by the regular motion of heavenly bodies — the sun, the moon, the stars — but this measurement is conventional, not natural. We choose certain periodic motions as standards, then apply them to duration.

“Duration in itself is to be considered as the fleeting continuance of the existence of things, which we represent to ourselves by the idea of a certain length, whereof we have no other perception than by the train of ideas that appear in our minds.”

A critical distinction arises between duration and expansion (space). Duration has only one dimension, a continuous succession; expansion has three dimensions, all coexisting. Yet both share the property of infinity, which Locke will examine separately. This chapter establishes that our idea of time, like space, originates entirely in sensation and reflection, with nothing innate about it.

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

Locke now compares the two great measures of existence — duration and expansion (space) — examining their similarities and differences. This chapter draws together the previous two discussions.

Both duration and expansion, Locke observes, are capable of greater and less. We speak of longer or shorter durations, larger or smaller spaces. Both admit of divisibility without end — we can imagine half an hour, then a quarter, then an eighth, and so on infinitely. Both are continuous quantities, not discrete. And both give us the idea of infinity, though in different ways.

The key difference lies in the nature of their parts. The parts of expansion are coexistent — all at once, simultaneous. The parts of duration are successive — one after another, never all existing together. This fundamental difference means that our ideas of space and time, though analogous in many respects, are ultimately distinct. We cannot apply the properties of one to the other without confusion.

“The parts of duration are not distinguishable one from another by any natural marks; nor can the beginning or ending of any part of it be known or distinguished, but as it is marked by some motion or change.”

Locke also addresses a common confusion: the idea of immensity applied to space and eternity applied to duration. These are both negative ideas — we arrive at them by denying any bounds or limits to space and time. The idea of eternity is not a positive comprehension of infinite duration, but the negation of beginning or end. Similarly, immensity is the negation of bounds to space. Both are constructed by the mind’s power of repeating its simple ideas without limit.

This chapter is important because Locke is carefully distinguishing two domains that philosophers had often conflated. By showing the different origins and structures of our ideas of space and time, he prepares the ground for his later critique of the idea of infinite duration and for his discussion of identity and diversity.

Book II: Of Ideas — Chapter 16: Of Number

Locke now examines number, which he considers the simplest and most universal of all simple modes. This chapter shows how the idea of number arises and how it becomes the foundation for all mathematical reasoning.

The idea of number, Locke argues, is given to the mind by sensation and reflection. We observe things — bodies, ideas, sounds — and we notice their distinctness. From this, we acquire the idea of unity or one. This is the simplest idea we have, and from it, by repeating the idea of unity, we generate all other numbers. The mind adds one to one to make two, adds another to make three, and so on.

Number is unique among simple modes in that its modes are the most distinct and precise. This is because each addition of one creates a completely new idea — two is not a modification of one, but a new, distinct idea. This precision makes number the foundation of demonstration and certainty. Mathematics achieves its certainty precisely because its objects — numbers — are perfectly determinate and their relations can be clearly known.

“Number applies itself to everything that exists or can be imagined, and therefore is the most general and extensive idea we have.”

Number also allows us to conceive of infinity, though only in a negative manner. We can never have a positive idea of infinite number because the mind cannot actually enumerate an endless series. What we have is the power to continue adding units without limit, and from this power we form the negative idea of number without end. Number, like duration and expansion, thus demonstrates both the power and the limits of human understanding.

Book II: Of Ideas — Chapter 17: Of Infinity

Locke devotes a full chapter to the idea of infinity, one of the most contested concepts in philosophy. He aims to clarify what we can and cannot mean by this term.

The idea of infinity, Locke argues, is not a positive idea of an endless magnitude. Rather, it is a negative idea — the negation of an end or limit. We acquire it by observing that the mind can repeat any idea of finite magnitude — space, duration, or number — without ever reaching a point where it must stop. From this power of endless addition, we form the idea of infinity.

However, Locke insists, we have no positive idea of infinite space or infinite duration. We cannot comprehend what it would mean for space to actually have no bounds, or for duration to have no beginning. What we call the idea of infinite space is really the idea of space plus the negation of bounds. This is why, Locke says, disputes about infinity so often become confused — people mistake the negative idea for a positive comprehension.

“We have no idea of infinite space, but only of a space that we can imagine to be extended without end, and that is all.”

Locke extends this analysis to God and eternity. When we say God is eternal, we mean his duration has no beginning or end — but this is not a positive comprehension of infinite duration. Similarly, when we say God is omnipresent, we mean his being extends through all space — but we do not positively grasp what this means. Our ideas of God’s infinity are thus necessarily negative and analogical, drawn from our finite experience.

This chapter serves as a corrective to philosophical overreach. Locke is showing that our idea of infinity, far from being a direct intuition of the infinite, is a negative construction based on our finite capacity for repetition. This limits what we can claim to know about the infinite.

Book II: Of Ideas — Chapter 18: Of Other Simple Modes

Locke now discusses several remaining simple modes that do not fall neatly under space, duration, or number. This chapter covers the modes of motion, sound, color, taste, and other qualities.

The simple mode of motion, Locke explains, is the idea of a body changing position in space. Its modes include speed and direction — fast, slow, straight, circular, and so on. All of these are modifications of the single simple idea of motion received from sensation. Similarly, sound has modes of pitch, volume, and timbre; color has modes of hue, saturation, and brightness; taste has modes of sweet, sour, bitter, and salt.

Locke notes that many of these modes are not as clearly distinguishable as those of number or extension. The boundaries between colors, for instance, are often vague — where exactly does blue become green? This imprecision makes these ideas less suited to demonstration and more subject to disagreement. Yet they are still genuine simple modes, produced by the mind’s power of combining and varying its simple ideas.

“The simple modes of motion are all the different degrees of swiftness, which are as many distinct ideas as are the different degrees of speed with which a body is moved.”

Locke also mentions the simple modes of power, particularly the power of producing change in things, which he will examine in detail later. For now, he simply notes that power is a simple idea of reflection, and its modes include active power (the ability to produce change) and passive power (the capacity to receive change).

This chapter fills out the taxonomy of simple modes, showing that while some modes admit of precise measurement (number, duration, extension), others are more qualitative and less precise. Yet all arise from the same basic operations of sensation and reflection.

Book II: Of Ideas — Chapter 19: Of the Modes of Thinking

Locke now examines the modes of thinking, which are simple modes of reflection. This chapter analyzes the various operations of the human mind.

The basic simple idea here is thinking itself, which we receive through reflection when we observe our own minds at work. The modes of thinking include perception, memory, remembrance, contemplation, attention, study, dreaming, and ecstasy. Each of these is a modification of the simple idea of thinking, distinguished by the mind’s degree of activity, focus, and awareness.

Locke divides thinking into two broad categories: active and passive. Passive thinking is what we call perception — when ideas are brought into the mind by sensation, the mind is largely receptive. Active thinking occurs when the mind deliberately directs its attention, compares ideas, combines them, or abstracts them. The modes of thinking thus vary along a spectrum from near-passive awareness to intense concentration.

“The mind, when it turns its view inward upon itself, and considers its own actions, has the idea of thinking; and from this idea, by varying it, it gets all the modes of thinking.”

Locke also addresses the question of whether the soul always thinks — a view held by some philosophers. He argues that it does not. Sensation and reflection show us that thinking is not constant; we sleep without dreaming, and even when awake, our thoughts often wander. The idea that the soul always thinks is not supported by experience, and experience is the only source of our ideas.

This chapter reinforces Locke’s empiricist method. Even the most intimate operations of the mind — thinking itself — are known only through reflection, not through innate knowledge. The modes of thinking are all constructed from the simple idea of thinking, which itself comes from experience.

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

Locke turns to the affective dimension of experience, examining pleasure and pain as simple ideas that accompany almost all other ideas.

Pleasure and pain, Locke argues, are simple ideas of both sensation and reflection. They accompany many of our other ideas — heat is sometimes pleasant, sometimes painful; thought can be delightful or distressing. These ideas are so fundamental that Locke calls them the hinges on which our passions turn. Every emotion, he claims, is a modification of pleasure and pain.

Locke lists the various passions as modes of pleasure and pain: love is pleasure in the thought of something; hatred is pain in the thought of something; desire is uneasiness at the absence of something pleasant; joy is pleasure arising from the possession of good; sorrow is pain at the loss of good; hope is pleasure at the prospect of future good; fear is uneasiness at the prospect of future evil; anger is uneasiness at perceived injury; and so on.

“Pleasure and pain are the two great springs of action in human life, and the ultimate ends of all our desires.”

Locke distinguishes between actual pleasure and pain (what we feel at present) and prospective pleasure and pain (what we anticipate). The latter gives rise to the passions, which are essentially anticipations of future good or evil. The will is determined by the greatest present uneasiness — a topic Locke will explore in Book II, Chapter 21.

This chapter is significant because it grounds all human motivation in these two simple ideas. There is no innate moral sense or eternal standard of good and evil — good is simply what produces pleasure, evil what produces pain. This has profound implications for Locke’s moral philosophy.

Book II: Of Ideas — Chapter 21: Of Power

This is one of the longest and most important chapters in the entire *Essay*. Locke examines the idea of power, which he considers a fundamental simple idea of reflection.

The idea of power arises, Locke argues, from observing changes in things. We see fire melt wax, the sun whiten cloth, and our own will move our bodies. From these observations, we acquire the idea of a power in one thing to produce change in another. The idea has two aspects: active power (the ability to produce change) and passive power (the capacity to receive change).

Locke applies this analysis to the human will. Volition or will is the power of the mind to direct its own thoughts or to move its body. Liberty or freedom is the power to act or not act according to one’s own choice. Locke defines freedom carefully: a free agent is one who has the power to do or forbear an action according to the determination of its own will. Freedom is not the same as volition — a man may will to jump off a cliff, but if he is bound, he lacks the power to execute his will and is therefore not free.

“Liberty, it is plain, consists in a power to do or not to do; to do or forbear doing as we will. This cannot be denied.”

The central debate of the chapter concerns determination of the will. Locke argues that the will is determined by the greatest present uneasiness. We are moved to act not by the greatest apparent good, but by the most pressing discomfort. A man may know that health is a greater good than drinking, but the immediate uneasiness of thirst determines his will. Locke calls this the “uneasiness of desire” — the felt lack of something absent that drives us to action.

Locke then introduces the concept of suspension of desire. The mature human being, he argues, has the power to suspend the execution of desire, to examine the object and weigh its consequences before acting. This suspension is the foundation of human freedom and moral responsibility. It allows us to consider long-term goods over immediate pleasures.

This chapter is crucial because it establishes Locke’s theory of human agency. The will is not free — it is determined by uneasiness — but the person is free because they can suspend action and deliberate. This distinction preserves both determinism and moral responsibility.

Book II: Of Ideas — Chapter 22: Of Mixed Modes

Locke now introduces a new category of complex ideas — mixed modes. These combine simple ideas from different sources into new, unified ideas.

Mixed modes, Locke explains, are complex ideas that combine several simple ideas of different kinds into one compound idea. They are distinct from ideas of substances (which represent real things) and simple modes (which vary a single idea). Examples include obligation, drunkenness, lie, murder, hypocrisy, and sacrilege. Each combines ideas of action, intention, circumstance, and result into a single concept.

Unlike ideas of substances, mixed modes are not intended to represent real essences. They are voluntary combinations made by the mind for its own purposes. We create the idea of a “lie” by combining ideas of speech, deception, and intention — but there is no single real being in nature that corresponds to “lie.” Mixed modes are archetypes of the mind’s own making, not copies of reality.

“The mind often exercises an active power in making these several combinations: for being once furnished with simple ideas, it can put them together in several compositions, and so make variety of complex ideas, without examining whether they exist so together in nature.”

Mixed modes are particularly important for moral language. Words like “virtue,” “vice,” “justice,” and “injustice” name mixed modes that we have constructed. The meaning of these terms depends on the precise combination of simple ideas we associate with them. This explains moral disagreements: people often attach different collections of simple ideas to the same moral term.

This chapter prepares the ground for Locke’s later discussion of the abuse of words (Book III). It shows that many philosophical and moral disputes arise from different definitions of mixed modes, not from genuine disagreement about facts.

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

Locke now examines the most important category of complex ideas — substances. This chapter addresses what we mean when we speak of a “thing” that underlies qualities.

The idea of substance, Locke argues, is a complex idea constructed by the mind. We observe that certain simple ideas — color, weight, hardness, fusibility — constantly go together. We come to regard them as belonging to one thing, and we give that thing a name — “gold,” “horse,” “man.” But we also suppose that there is some substratum or support in which these qualities inhere, which we call “substance in general.”

Locke famously calls this substratum a “something, I know not what.” We have no clear and distinct idea of substance apart from its qualities. The idea of substance is nothing more than an obscure and relative idea of a supposed support for qualities we find existing together. This is one of Locke’s most radical claims: the idea of substance, which had been central to Aristotelian metaphysics, is actually a vague and uncertain notion.

“The idea that belongs to the word ‘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.”

Locke distinguishes between three kinds of substances: material substances (bodies), spiritual substances (minds or souls), and God (the supreme spirit). Our ideas of each are complex — we attribute to matter the powers of solidity, extension, and motion, and to spirit the powers of thinking, willing, and knowing. But our ideas of both are ultimately based on the qualities we observe, not on any direct insight into their inner natures.

This chapter is a critical intervention in metaphysics. Locke demystifies the concept of substance, showing that it is not a direct intuition of reality but a human construction to account for the regular conjunction of qualities. He thereby limits what we can claim to know about the ultimate nature of reality.

Book II: Of Ideas — Chapter 24: Of Collective Ideas of Substances

Locke examines a special type of complex idea — collective ideas of substances. These are ideas formed by grouping multiple individual substances together under one name.

A collective idea is the idea of several substances considered together as a single thing. Examples include an army (many men), a flock (many sheep), the universe (all bodies), and a fleet (many ships). Each of these is a complex idea formed by combining many individual ideas of substance into one unified concept.

Locke emphasizes that these collective ideas are as much ideas of substance as individual ones. The mind treats the collection as a single entity, gives it a name, and predicates qualities of it — we say “the army is powerful” or “the fleet is swift.” The reality of the collective idea, however, lies in the individual substances that compose it, not in any additional entity.

“The mind, by its power of composition, can join together the ideas of several distinct substances into one idea, and thus make a complex idea of a multitude of substances, which is as much one idea as the idea of a man.”

This chapter reinforces Locke’s nominalism. There is no real essence of “army” beyond the individual men who compose it. The unity of the collective idea is a mental construction, not a metaphysical reality. This applies also to complex ideas of substances like “man” and “horse” — their unity is similarly constructed, though based on natural conjunctions of qualities.

Book II: Of Ideas — Chapter 25: Of Relation

Locke now turns to relation, one of the mind’s fundamental ways of generating new ideas. This chapter opens a series that will examine various types of relations.

Relation, Locke explains, is a complex idea that arises when the mind compares one idea with another. It consists in considering two things together and noticing how they stand with respect to each other. Every relation has three components: the subject (the thing related), the term (the thing to which it is related), and the foundation (the basis of the comparison).

Relations are distinct from the ideas of the things themselves. A father is a man, but “father” adds the relation of paternity to another being. Relation, Locke emphasizes, is a product of the mind’s comparing activity — it is not a quality inherent in things themselves. The same thing can enter into many different relations without changing its own nature.

“Besides the ideas of things, whether particular or general, the mind has a power of comparing them one with another, and thus making complex ideas of relation.”

Locke classifies relations by their foundation. Some are based on cause and effect (father to son, creator to created), others on time (older, younger, contemporary), others on place (above, below, distant), and so on. The most important relations for human life are moral relations (based on conformity to law) and natural relations (based on real connections in nature).

This chapter establishes relation as a creative act of the mind, not a passive reception of reality. This will be important for Locke’s later arguments about moral and political philosophy, where relations like “right,” “duty,” and “obligation” are products of comparison and judgment.

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

Locke now examines one of the most fundamental relations — cause and effect. This chapter analyzes how we arrive at this relation and what it means.

The ideas of cause and effect, Locke argues, arise from our observation of change. We notice that one thing — fire, for example — produces a change in another — wax melting. From this, we acquire the idea of cause as that which produces any change, and effect as the change produced. The relation of cause and effect is thus based on the simple ideas of power, change, and succession.

Locke distinguishes between different types of causes. A material cause is the substance out of which something is made; an efficient cause is the agent that produces the effect; a final cause is the end or purpose for which something is done; a formal cause is the essence or form of the thing. However, Locke focuses primarily on efficient causation — the production of change by an active power.

“That which produces any simple or complex idea, we denote by the general name ’cause’; and that which is produced, by the name ‘effect.'”

Locke notes that our knowledge of causation is limited. We observe that a certain event regularly follows another — fire regularly melts wax — but we do not directly perceive the power that produces the effect. We see the constant conjunction, not the necessary connection. This is a crucial point that David Hume would later develop: causation is known through experience of regular succession, not through insight into necessary connections.

This chapter limits the pretensions of causal reasoning. We can know that certain things are regularly conjoined, but we cannot know the inner necessity that connects cause and effect.

Book II: Of Ideas — Chapter 27: Of Identity and Diversity

This is perhaps the most famous chapter in Book II. Locke examines the ideas of identity and diversity, particularly as they apply to persons.

Identity, Locke argues, depends on the principle of individuation — what makes a thing the same thing over time. For any being, identity consists in the continuation of the same life or existence through time. For a plant, identity is the continuation of the same organic life; for an animal, the same living body; for a person, the same consciousness.

Locke famously distinguishes between man (a human organism) and person (a thinking intelligent being with consciousness). A man is the same man as long as the same living body continues, even if consciousness is interrupted by sleep or forgetfulness. A person, however, is the same person as long as the same consciousness continues — the same ability to reflect on past actions and recognize them as one’s own.

“For as far as any intelligent being can repeat the idea of any past action with the same consciousness it had of it at first, and with the same consciousness it has of any present action, so far it is the same personal self.”

Locke draws crucial consequences from this. Personal identity — what makes me the same person I was yesterday — is founded on consciousness, not on the same body or the same soul. If the soul of a prince, with all its memories and consciousness, were transferred into the body of a cobbler, the person would be the prince, not the cobbler. This means that personal identity is necessary for moral responsibility — we can only be held accountable for actions that we are conscious of having performed.

This chapter revolutionized philosophical discussions of identity. Locke’s criterion of consciousness-based personal identity remains central to debates in metaphysics and ethics today.

Book II: Of Ideas — Chapter 28: Of Other Relations

Locke continues his examination of relation, focusing on moral relations and their foundations.

The most important relations for human life, Locke argues, are those that involve moral good and evil. Moral good and evil are defined by their relation to law. An action is morally good if it conforms to a law; morally evil if it violates a law. Locke identifies three types of law to which actions are related:

1. The divine law — the law of God, revealed through nature or scripture, which determines actions as sins or duties. 2. The civil law — the law of the state, enforced by penalties, which determines actions as criminal or lawful. 3. The law of opinion or reputation — also called the law of fashion or private censure, which determines actions as virtues or vices.

“Good and evil, as has been shown, are nothing but pleasure and pain; and that which is morally good or evil is only the conformity or disagreement of our voluntary actions to some law, whereby good or evil is drawn on us by the will and power of the law-maker.”

The law of opinion is particularly important for understanding society. By praising certain actions and condemning others, communities create a system of moral evaluation that governs behavior. Virtue and vice, in this sense, are relative to the community’s standards. This law is enforced by the natural human desire for praise and fear of blame.

Locke emphasizes that these three laws often overlap — what violates divine law also tends to violate civil law and the law of opinion — but they are distinct. An action may be legal under civil law but sinful under divine law, or vice versa.

This chapter grounds morality in law and social practice, not in innate ideas. Moral relations are constructed by comparing actions to standards — divine, civil, or social — and these standards themselves are known through experience.

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

Locke now examines the qualities of ideas themselves — their clarity, obscurity, distinctness, and confusion. This chapter helps the reader evaluate the quality of their own understanding.

Clear and obscure ideas, Locke explains, refer to the liveliness or vividness of an idea. A clear idea is one that is present to the mind with full force and intensity, like the perception of sunlight. An obscure idea is faint and weak, like the memory of a past perception. Clarity and obscurity are matters of degree, determined by the freshness and force of the original impression.

Distinct and confused ideas refer to the boundaries between ideas. A distinct idea is one that the mind can separate from all other ideas — we can tell precisely where it begins and ends. A confused idea is one that runs into other ideas without clear boundaries, so that we cannot say exactly what it includes and excludes.

“The memory of some things is very clear, and of others very obscure; and oftentimes the ideas of the same thing, which we have at different times, are more or less clear and distinct.”

Locke applies these distinctions to philosophical disputes. Many arguments, he suggests, arise from confused ideas — people use words without having distinct ideas corresponding to them. The remedy is to examine our ideas carefully, ensuring that they are both clear (vivid) and distinct (well-bounded) before we reason about them.

This chapter functions as a diagnostic tool. Locke is teaching his readers how to recognize when their own ideas are inadequate, and thus how to avoid the errors that come from reasoning with confused or obscure notions.

Book II: Of Ideas — Chapter 30: Of Real and Fantastical Ideas

Locke introduces another important distinction among ideas — real versus fantastical ideas.

Real ideas, Locke explains, are those that have a foundation in nature. They conform to something actually existing outside the mind, or to the actual operations of the mind itself. The idea of a horse is real because horses exist; the idea of heat is real because heat is actually produced in us by external objects.

Fantastical ideas, by contrast, are those that have no foundation in reality. They are chimeras — inventions of the mind that do not correspond to anything actual. The idea of a centaur (half-man, half-horse) is fantastical because no such being exists. The idea of a golden mountain is fantastical, even though gold and mountains exist separately.

“By real ideas, I mean such as have a foundation in nature; such as have a conformity with the real being and existence of things, or with their archetypes.”

Locke notes that fantastical ideas are not necessarily false or useless — poets and painters create them for artistic purposes. But when we mistake fantastical ideas for real ones in philosophy or science, we fall into error. The distinction helps us evaluate whether our ideas refer to actual things or are merely mental constructions.

This chapter reinforces Locke’s commitment to empiricism. The reality of an idea is measured by its correspondence to actual experience, not by its internal coherence or beauty.

Book II: Of Ideas — Chapter 31: Of Adequate and Inadequate Ideas

Locke continues his taxonomy of ideas by distinguishing adequate from inadequate ideas.

Adequate ideas, Locke argues, are those that perfectly represent their archetypes — they contain all the simple ideas that belong to the thing. Simple ideas are always adequate because they are exact copies of the qualities that produce them. The idea of whiteness, for example, perfectly represents the quality of whiteness as it affects our senses.

Inadequate ideas are those that do not fully represent their archetypes. Nearly all our complex ideas of substances are inadequate. Our idea of gold, for instance, includes only a few of the qualities that actually belong to gold — its color, weight, malleability, and perhaps its solubility in aqua regia. But gold has many more qualities that we have not discovered — and perhaps some we cannot discover. Our idea is therefore incomplete.

“Our complex ideas of substances are all inadequate, because they are not exactly conformable to the real essences of things, but are only collections of those qualities which are found to exist in them.”

Modes and relations are different. Our ideas of modes — like triangle or gratitude — are adequate because they are our own creations. We define what a triangle is, and any triangle we conceive must conform to that definition. Similarly, our ideas of relations are adequate when we clearly understand the relation we are considering.

This chapter highlights a fundamental limitation of human knowledge. We cannot have adequate ideas of substances because we do not know their real essences — the underlying structures that produce their observable qualities. Our knowledge of the natural world is necessarily incomplete.

Book II: Of Ideas — Chapter 32: Of True and False Ideas

Locke examines the application of the terms true and false to ideas. Strictly speaking, he argues, ideas themselves are not true or false.

Truth and falsehood, Locke insists, are properties of propositions, not of ideas. A proposition is an affirmation or denial — “the sun is shining” or “gold is yellow.” Truth consists in the agreement of words with the ideas they stand for, or of ideas with reality. An idea considered by itself — the idea of a circle, for example — is neither true nor false; it simply is what it is.

However, Locke concedes that we commonly call ideas true or false in a secondary sense. An idea is called true when it conforms to something beyond itself — when our idea of a substance corresponds to the real qualities of that substance. An idea is called false when we mistake it for something else — when, for example, we mistake the idea of a chimera for the idea of a real being.

“Though ideas themselves be not true or false, yet when the mind refers them to anything else, then they may be true or false, in the sense of being conformable or not conformable to that thing.”

Locke identifies several ways that ideas become false in this secondary sense: when we judge that our ideas correspond to the real essences of substances (which we cannot know), when we mistake fantastical ideas for real ones, or when we confuse inadequate ideas for adequate ones.

This chapter refines the epistemological framework. By showing that truth belongs primarily to propositions, Locke prepares the ground for Book IV, where he will examine the nature and extent of human knowledge.

Book II: Of Ideas — Chapter 33: Of the Association of Ideas

The final chapter of Book II introduces a concept that will be enormously influential — the association of ideas. This chapter is a late addition to the *Essay*, responding to objections that Locke’s account of ideas did not explain irrational beliefs and behaviors.

Association of ideas, Locke explains, is the process by which ideas become linked in the mind through repeated experience, creating connections that are not based on reason. Some connections are natural — fire and heat, for example, are connected by the natural order of cause and effect. But other connections are accidental, formed by chance or custom.

These accidental associations explain many irrational phenomena. A child who is frightened by a spider in a dark room may forever associate darkness with fear. A person who hears a false argument repeatedly may come to believe it through sheer repetition. The association of ideas, Locke says, is the “great source of errors, prejudices, and superstitions.”

“Some of our ideas have a natural correspondence and connection one with another; it is the office and excellency of our reason to trace and observe that connection. But there is another connection of ideas wholly owing to chance or custom; ideas that in themselves are not at all of kind, come to be so united in some men’s minds that it is very hard to separate them.”

Locke gives vivid examples: a man who learned dancing in a room with a trunk may never be able to dance without looking at that spot; a person cured of a fever by a ridiculous charm may forever believe in the charm’s power. These associations are formed in childhood and become deeply entrenched, resisting all rational argument.

This chapter anticipates modern psychology and provides a naturalistic explanation for many human irrationalities. It also completes Book II’s account of the human mind, showing that even our errors arise from the same mechanisms — sensation, reflection, and association — that produce our knowledge.