Preface by the editor. Life of the author. Analysis of Mr. Locke’s doctrine of ideas [fold. tab.] Essay concerning human understanding. Book I-book III, chap. VI – John Locke

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

Editor’s Preface

The volume opens with a preliminary note from the editor, situating Locke’s text within the broader context of eighteenth-century philosophical publishing. The editor explains that the present edition includes not only the complete text of the *Essay* but also a prefatory life of the author and a critical analysis of Locke’s doctrine of ideas, designed to assist the reader in following the work’s arguments. The editor remarks that Locke’s treatise had already been widely circulated and debated, and that this edition aims to present it in a form both accurate and accessible. The editor also notes the inclusion of a fold-out tab for reference, indicating the work’s use as a study text.

“The following sheets contain a faithful reprint of the last edition corrected by the author himself.”

This editorial framing matters because it establishes the text as a definitive version of Locke’s thought. The reader is assured that what follows is authoritative, and the inclusion of supplementary material signals that the *Essay* was understood even at the time of its publication as a work requiring careful exposition. The editor’s role is minimal but purposeful: to present Locke’s own words without interference, while adding contextual aids.

The Life of the Author

This prefatory biography recounts Locke’s life from his birth in 1632 to his death in 1704. The reader learns that Locke was educated at Westminster School and Christ Church, Oxford, where his early studies in scholastic philosophy left him dissatisfied. The biography emphasizes Locke’s turn toward medicine and natural philosophy, his association with Lord Ashley (later the Earl of Shaftesbury), and his exile in Holland during political turmoil. It was during this exile, the biography states, that Locke composed the first drafts of the *Essay*, returning to England after the Glorious Revolution to publish it in 1690.

The biography presents Locke as a man of cautious temper, devoted to inquiry and suspicious of dogma. It notes his friendships with Isaac Newton and Robert Boyle, and his involvement in practical affairs as a commissioner of trade. The narrative stresses that Locke’s philosophical work emerged not from academic speculation but from practical discussions among friends, who found themselves entangled in disputes that seemed to require a prior examination of the human understanding itself. The biography concludes with an account of Locke’s retirement at Oates in Essex, where he revised the *Essay* through successive editions until his death.

This life-story is not merely ornamental. It establishes the *Essay* as a work born from lived intellectual struggle, not from abstract system-building. The reader is prepared to encounter a text that is empirical in spirit, skeptical of received authority, and grounded in the concrete limits of human cognition.

Analysis of Mr. Locke’s Doctrine of Ideas

Before the text of the *Essay* itself, the editor inserts a critical analysis of Locke’s central theory. This analysis is presented as a fold-out tab, intended to be consulted alongside the reading. It breaks down Locke’s doctrine of ideas into its principal components: the origin of ideas in sensation and reflection, the distinction between simple and complex ideas, the nature of abstract ideas, and the limits of knowledge.

The analysis explains that Locke defines idea as “whatsoever is the object of the understanding when a man thinks.” All ideas, Locke holds, come from one of two sources: sensation (external objects affecting the senses) or reflection (the mind’s perception of its own operations). From these simple ideas, the mind builds complex ones through combination, comparison, and abstraction. The analysis highlights Locke’s critique of innate ideas — the claim that certain principles are imprinted on the mind at birth — as the foundational move of the entire *Essay*.

“There is nothing in the understanding that was not first in the senses.”

This analysis is crucial because it gives the reader a roadmap for the argument to come. It makes explicit that Locke’s entire epistemology rests on the rejection of innate knowledge and the elevation of experience as the sole source of ideas. The analysis does not evaluate Locke’s position but rather clarifies its structure, ensuring that the reader enters the *Essay* itself with a grasp of its core vocabulary.

Book I: Of Innate Notions — Chapter I: The Introduction

We begin the *Essay* proper. The Introduction sets out Locke’s purpose: to inquire into the origin, certainty, and extent of human knowledge, together with the grounds and degrees of belief, opinion, and assent. Locke states that he does not aim at a complete system but at a modest examination of the understanding itself, to see what objects it is and is not fitted to handle. He calls this a “historical, plain method” — an investigation into how the mind actually comes by its ideas, not a metaphysical deduction of what it ought to contain.

Locke opens by observing that many disputes among philosophers arise not from ignorance of the subject matter but from misunderstanding the powers of the mind. “If we can find out,” he writes, “how far the understanding can extend its view, how far it has faculties to attain certainty, and in what cases it can only judge and guess, we may learn to content ourselves with what is attainable by us.” He suggests that a candle set up within us may light us as far as it can reach, and that we need not extinguish it in order to seek a sun beyond our horizon.

This passage presents the modesty of Locke’s project. He is not claiming that human knowledge is impossible or worthless, only that it has limits. The understanding, like a finite instrument, can do certain things well and others not at all. The task of the *Essay* is to map those limits, so that we may avoid fruitless speculation and focus on what we can genuinely know. The Introduction thus sets the tone: empirical, cautious, and practical.

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

Locke now launches his attack on the doctrine of innate principles. He takes as his target the claim—common among some ancient and medieval philosophers, and given new life in the rationalist systems of his own time—that there are certain speculative and practical principles imprinted on the human mind from birth. The most common examples cited are the principle of identity (“Whatever is, is”) and the principle of non-contradiction (“It is impossible for the same thing to be and not to be”).

Locke argues that if such principles were truly innate, they would be universally known and assented to. Yet we find that children, idiots, and those without education do not have the slightest awareness of these principles. “It seems to me a near contradiction,” he writes, “to say that there are truths imprinted on the soul which it perceives or understands not.” If the mind does not actually think of these principles, it cannot be said to have them. To claim that they are present though unperceived is to empty the notion of “imprinted” of all meaning.

“There is nothing more commonly taken for granted than that there are certain principles, both speculative and practical, universally agreed upon by all mankind; which therefore, they argue, must needs be the constant impressions which the souls of men receive in their first beings.”

Locke further examines the supposed universal consent. He observes that different peoples, and even different individuals within the same society, hold widely varying moral and speculative beliefs. There is no principle, he claims, to which all humans give their assent. Even the law of self-preservation, often cited as a universal practical principle, is violated by some who choose death over dishonor. Universal consent, therefore, cannot serve as evidence for innate principles.

The argument here is straightforward but devastating to the innate idea hypothesis. Locke shows that the appeal to universal agreement is empirically false, and that the notion of unperceived innate ideas is conceptually incoherent. The mind, he insists, begins as a tabula rasa — a blank slate — and acquires all its content from experience. The rest of the *Essay* will build on this foundation.

Book I: Of Innate Notions — Chapter III: No Innate Practical Principles

Locke extends his critique to practical principles, particularly moral rules. If there were innate moral principles, he argues, they would be universally recognized and enforced. Yet we find that different cultures, and even different eras, have endorsed contradictory moral codes. What is considered virtuous in one place is vicious in another. Even the most widely accepted moral rule—that parents should care for their children—is violated in some societies where children are abandoned or killed.

Locke acknowledges that some moral rules appear nearly universal, such as the condemnation of murder and theft. But he argues that these can be explained by self-interest and social utility, not by innate imprinting. Human beings naturally discover that their own security and happiness depend on rules of justice and peace, and they therefore teach these rules to their children. But such teaching is a matter of education, not of innate endowment.

“He that will carefully peruse the history of mankind, and look abroad into the several tribes of men, will be able to satisfy himself that there is not that principle of morality which is not rejected somewhere or other by some whole society.”

Locke also considers the objection that conscience proves the existence of innate moral principles. He responds that conscience is not an innate faculty but a product of education and custom. What we call conscience is simply our own judgment about the moral quality of our actions, and that judgment is shaped by the beliefs we have been taught. A person raised in a society that approves of revenge will feel no guilt in taking it.

This chapter deepens the critique by showing that moral claims are especially resistant to the innate idea hypothesis. If even the most basic moral principles vary across time and place, then they cannot be imprinted by God or nature on every human mind. Morality, like speculative knowledge, must be learned from experience.

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

Locke concludes Book I by addressing residual objections. Some philosophers have claimed that innate principles are present in the mind as a capacity or disposition, not as actual thoughts. Locke rejects this move as a trivialization: if “innate” means only that the mind is capable of learning certain truths when exposed to experience, then everything is innate, and the term loses its meaning. The capacity to know is indeed universal, but that capacity is not the same as having the knowledge itself.

“If the capacity of knowing be the natural impression contended for, all the truths a man ever comes to know will, by this account, be every one of them innate.”

Locke also examines the claim that certain principles are innate because they are universally assented to as soon as they are understood. His response is that this universal assent, when it occurs, is due not to innateness but to the self-evidence of the propositions themselves. Once a person has grasped the meaning of “Whatever is, is,” he cannot help but assent to it — but that is because the proposition is evident from the terms themselves, not because it was pre-imprinted. The same could be said of any mathematical axiom: once the terms are understood, the truth is perceived.

Book I thus concludes with a thorough demolition of the innate idea hypothesis. Locke has shown that it lacks empirical support, that it rests on equivocal definitions, and that it offers no explanatory advantage over the simple claim that all knowledge comes from experience. The stage is now set for Book II, where Locke will present his positive account of the origin of ideas.

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

We now enter the heart of Locke’s positive project. This chapter serves as the foundation for everything that follows: Locke establishes where all ideas come from, answering the question he posed in the Introduction — how does the mind come to be furnished with its contents?

Locke begins by stating that every person is conscious of having ideas in his mind, and that these ideas are the materials out of which all knowledge is built. The central question, then, is: whence has the mind all the materials of reason and knowledge?

His answer is delivered in a single, decisive stroke: “To this I answer, in one word, from EXPERIENCE. In that all our knowledge is founded; and from that it ultimately derives itself.” This is the famous empiricist principle that will define the entire work. Locke immediately qualifies that he means all the materials of thinking — not the faculty of thinking itself, which he takes to be innate in the sense that humans are born with the capacity to receive ideas and operate upon them.

Locke then introduces his two great sources of ideas. The first is sensation: our senses, directed at particular sensible objects, convey into the mind distinct perceptions of things such as yellow, white, heat, cold, soft, hard, bitter, sweet. He calls this “the great source of most of the ideas we have, depending wholly upon our senses, and derived by them to the understanding.” This source supplies the mind with all its ideas of external objects and their qualities.

The second source is reflection: the perception of the operations of our own minds as we employ it about the ideas it has received from sensation. Locke describes this as “the perception of the operations of our own minds within us, as it is employed about the ideas it has got.” These operations — perceiving, thinking, doubting, believing, reasoning, knowing, willing — produce ideas that cannot be had from external objects alone. Reflection might be called “internal sense,” but Locke is careful to distinguish it from sensation proper.

Locke emphasizes that these two — sensation and reflection — are “the only windows by which light is let into this dark room.” The mind, at birth, is like a closet wholly shut from light, with only these two inlets for the ideas that will furnish it. All the vast store of human knowledge, no matter how abstract or complex, can be traced back eventually to these two sources. Even ideas like God, infinity, or substance are ultimately constructed by the mind combining, repeating, or comparing ideas originally derived from sensation or reflection.

What this means is that Locke has now fully articulated his alternative to the innate idea theory. Instead of pre-imprinted principles, the mind is a tabula rasa — a blank slate — at birth, and all its content comes through experience. The metaphor of the “dark room” or “empty cabinet” is central: the understanding is empty until sensation and reflection furnish it. This chapter is the hinge upon which the entire Essay turns — having cleared away innate ideas in Book I, Locke now provides the positive foundation for his entire epistemology.

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

Having established the two sources of ideas, Locke now turns to classify them. This chapter introduces the fundamental distinction between simple ideas and complex ideas, which will structure the whole of Book II.

Simple ideas, Locke explains, are “the materials of all our knowledge” — they enter the mind through sensation and reflection as unmixed, uniform appearances in the mind. The mind is passive in receiving them: it cannot make or destroy simple ideas any more than a mirror can refuse or alter the images placed before it. When the eye sees the color white, or the tongue tastes sugar, the mind receives that simple idea exactly as it is given.

Locke offers an important analogy: “the understanding is merely passive in the reception of all its simple ideas.” Just as the body cannot refuse to feel heat when placed near a fire, the mind cannot refuse to have the idea of heat when the sensation is present. This passivity is crucial for Locke’s epistemological project — it grounds our confidence that simple ideas are real and trustworthy, given directly by experience.

These simple ideas come in several varieties. Some come into the mind by one sense only: colors by sight, sounds by hearing, tastes by taste, smells by smell, and tangible qualities like heat and cold by touch. Others come by more than one sense: space, extension, figure, rest, and motion are perceived by both sight and touch. Still others come only by reflection: perception and willing are operations of the mind that we know only through internal sense. Finally, some simple ideas come by both sensation and reflection: pleasure, pain, power, existence, and unity are suggested to the mind by both external and internal experience.

Locke stresses that these simple ideas are “the beginning and foundation of all our knowledge.” They cannot be invented by the mind; they must be given from experience. A blind man can never have the idea of colors, nor a deaf man the idea of sounds. The mind has no power to frame any new simple idea not received through these channels. This is Locke’s fundamental constraint on human understanding: we can combine, compare, and abstract simple ideas indefinitely, but we cannot create new ones from nothing.

This classification establishes the first principle of Locke’s way of ideas: all complex ideas — whether of substances, modes, or relations — are ultimately composed of simple ideas received from sensation or reflection. If any proposed idea cannot be traced back to these sources, it is suspect. This becomes Locke’s main weapon against metaphysical speculations that outrun experience.

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

This chapter contains one of the most famous and influential distinctions in the entire Essay — the distinction between primary qualities and secondary qualities. Locke introduces this distinction to explain how our simple ideas relate to the objects that produce them.

Locke begins by stating that bodies produce ideas in us through impulse — the only way bodies can operate on one another. When we perceive an object, insensible particles of matter strike our senses, and this motion is transmitted through the nerves to the brain, producing the sensation in the mind. But what is the relationship between the qualities in the object and the ideas in our minds?

Primary qualities are “utterly inseparable from the body, in what estate soever it be.” These include solidity, extension, figure, motion or rest, and number. Locke says that a grain of wheat may be divided and re-divided, but its parts will always have solidity, extension, figure, and mobility. These qualities are in the objects themselves, whether we perceive them or not. Our ideas of primary qualities are resemblances — they actually resemble the qualities in the object.

Secondary qualities, on the other hand, are “nothing in the objects themselves but powers to produce various sensations in us by their primary qualities.” Colors, sounds, tastes, smells, and tactile sensations like heat and cold are not actually in the objects. The idea of redness is not a resemblance of anything in a rose; it is produced in our minds by the size, figure, and motion of the rose’s minute particles. The secondary quality is a power in the object — a power to produce an idea in us — not a quality that the object possesses as we perceive it.

“What is sweet, blue, or warm in idea, is but the certain bulk, figure, and motion of the insensible parts in the bodies themselves, which we call so.”

Locke offers the example of fire and the sensation of pain. When we put our hand near a fire, the fire produces both the idea of heat and the sensation of pain. But we do not suppose that the pain is actually in the fire — it is merely an effect of the fire’s operation on us. Locke argues that the same reasoning applies to heat, color, and sound: they are no more in the object than pain is.

Locke further distinguishes a third category: tertiary qualities or powers. These are the powers in objects to produce changes in other objects, such as the power of fire to melt wax. These are not simple ideas at all, but rather complex relations that the mind observes.

This distinction matters enormously for Locke’s epistemology. It means that our ideas of secondary qualities are not direct representations of the world as it is — they are subjective effects produced by the real primary qualities of objects. The world as it exists independently of us contains only particles with size, shape, motion, and solidity. The colors and sounds we experience are added by our sensory apparatus. This gives Locke a powerful tool for analyzing the limits of human knowledge: we can know primary qualities as they are, but secondary qualities exist only as powers relative to our perception.

Book II: Of Ideas — Chapter IX: Of Perception

Locke now turns to the faculty by which the mind receives its ideas — perception. This is the first operation of the mind upon its ideas and, according to Locke, the first step toward knowledge.

Perception, Locke says, “is the first faculty of the mind exercised about our ideas.” It is the passive reception of ideas into the understanding through sensation. When the eye sees light or the ear hears sound, that very reception is perception. Locke insists that perception is “the inlet of all knowledge” and that it is “the first and simplest idea we have from reflection.”

But perception is not merely the physical impact of objects on the senses. Locke distinguishes between the physiological process — the “impression on the organ” — and the perception itself, which is the idea in the understanding. He notes that a person may have his eyes open in the dark and receive no ideas, or may have an object before his eyes but fail to attend to it. In such cases, there is sensory stimulation without perception. Perception requires the mind to actually take notice of the impression.

Locke offers a striking example concerning perception in infants. Before a child is born, it presumably receives many sensations — perhaps the warmth of the womb, the pressure of its position — but we have no reason to think the child perceives these as ideas until it has developed the capacity to attend to them. Perception, for Locke, is not automatic; it requires the mind to be awake and active in receiving.

He also addresses the question of whether perception is found in animals. Locke answers that beasts certainly have perception — “I doubt not but that beasts have perception” — and that some of them surpass humans in the sharpness of their senses. The difference between human and animal perception, for Locke, is not in the faculty of perception itself but in the higher operations the human mind can perform upon the ideas it receives.

This chapter lays the groundwork for understanding how knowledge begins. Perception is the gate — it is through perception that simple ideas enter the mind. Without perception, no ideas; without ideas, no knowledge. The chapter reinforces Locke’s fundamental commitment: knowledge begins with and is limited by what we can perceive through sensation and reflection.

Book II: Of Ideas — Chapter X: Of Retention

The next faculty Locke examines is retention — the power of the mind to keep ideas that have been received through perception. Retention comes in two forms: contemplation and memory.

Contemplation is “keeping an idea which is actually in view for some time.” When we look at a visible object and continue to attend to it, we are retaining it through contemplation. This is the simplest form of retention — the mind merely holds onto an idea it is actively perceiving.

Memory is “the power to revive again in the mind those ideas which, after imprinting, have disappeared, or have been as it were laid aside out of sight.” Memory allows us to bring back ideas that are no longer present to the senses. Locke calls this “the storehouse of our ideas” — without memory, all our knowledge would be limited to the present moment, and we could never engage in reasoning, comparison, or abstraction.

Locke lists several important features of memory. First, ideas can be retained for varying lengths of time — some are fixed “with so much strength that they can never be lost,” while others fade quickly. Second, the speed and accuracy of memory vary greatly between individuals and at different ages. Third, memory depends on attention and repetition — ideas that are frequently revisited are more firmly retained.

Locke also discusses memory disorders and their effects on knowledge. He notes that some people lose their memory entirely for certain periods of their lives, or lose it gradually with age. In such cases, the person may still perceive new ideas but cannot connect them with past experience — “the man is, as it were, a child again, as to his knowledge.”

The chapter concludes with a reflection on the indispensability of memory for personal identity and knowledge. Without memory, there would be no consciousness of duration, no recognition of past events, no ability to learn from experience. Memory is what allows the simple ideas received through perception to become the permanent materials of knowledge, capable of being examined, combined, and compared at will.

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

Locke now examines the higher operations the mind performs upon its simple ideas — the discerning faculty and its related powers. These operations transform the raw materials of sensation and reflection into the complex ideas that constitute human knowledge.

The first and most fundamental operation is discerning — “the faculty whereby the mind perceives the difference and agreement of any of its ideas.” Locke calls this “the first step towards knowledge” because without it, we could never distinguish one thing from another. A person who cannot discern between ideas cannot judge, reason, or understand. This faculty is what separates humans from beasts and from the merely passive reception of impressions.

Next is comparing — “the examining the ideas one with another, and measuring their agreement or disagreement.” Comparison is the basis of all relations, such as cause and effect, identity and diversity, and proportion in quantity. Locke notes that beasts seem to have very little capacity for comparison, while it is “one of the principal sources of human knowledge.”

Compounding is the power “to put together several simple ideas into one compound one, and thus to make complex ideas.” This is the operation by which we form ideas of substances (e.g., “man,” “horse,” “gold”), modes (e.g., “beauty,” “gratitude,” “murder”), and relations. Through compounding, the mind builds all the complex furniture of thought from the simple materials given by experience.

Abstraction is perhaps the most important operation for Locke, as it is the faculty that “puts the difference betwixt man and beasts.” Abstraction is “the separating of the ideas from all particular circumstances of time and place, and from all other ideas that accompany them in their existence.” This allows the mind to form general ideas — for example, the idea of “man” as such, abstracted from any particular individual. General ideas are the foundation of language and rational discourse.

“Words become general by being made the signs of general ideas; and ideas become general by separating from them the circumstances of time and place, and any other ideas that may determine them to this or that particular existence.”

Locke adds a discussion of naming: when we form complex ideas, we usually give them a name, and the name then stands for that combination of simple ideas. This is the origin of language and the foundation of all classification. The chapter closes with Locke emphasizing that these faculties — discerning, comparing, compounding, and abstracting — are what raise humans above animals and make science and reasoning possible.

This chapter is essential for understanding Locke’s constructivist theory of mind: the understanding does not passively receive ready-made complex ideas; it actively builds them from simple components using these operations. All the rich content of human thought — our concepts of substances, our moral ideas, our mathematical knowledge — is the product of the mind working upon the simple ideas given by experience.

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

We are now at the point where Locke formally introduces the distinction between simple ideas and complex ideas, a distinction that structures the entire remainder of Book II.

Having explained in previous chapters how the mind receives simple ideas from sensation and reflection, and how it performs operations like discerning, comparing, and abstracting, Locke now presents the third crucial faculty: the power to combine simple ideas into complex ideas. This chapter functions as a bridge — it defines what complex ideas are, lists their three main sorts, and announces the plan for the rest of the book. “As simple ideas,” Locke writes, “are observed to exist in several combinations united together, so the mind has a power to consider several of them united together as one idea; and that not only as they are united in external objects, but as itself has joined them together.” This power of composition is what produces all the variety of human thought beyond the raw materials of sensation and reflection.

Locke defines complex ideas as those made by the mind “by repeating, comparing, and uniting” simple ideas. He emphasizes that the mind can create new combinations that have “no union in nature” — it can join a horse’s head to a human body, producing the idea of a centaur. However, the materials it uses are always simple ideas that came originally from sensation or reflection. The mind cannot invent new simple ideas; it can only recombine those it has received. Having established this, Locke then divides complex ideas into three categories: modes, substances, and relations. Modes are complex ideas that “do not contain in them the supposition of subsisting by themselves” — they are dependent on substances, like the idea of a triangle or gratitude. Substances are those combinations of simple ideas that “are taken to represent distinct particular things subsisting by themselves,” like a man, a sheep, or gold. Relations are ideas that arise from “the comparing of one idea with another.”

This classification is not arbitrary; it reflects Locke’s fundamental conviction that the mind’s power to combine simple ideas follows logical principles grounded in how we experience the world. We experience qualities existing together in things (substances), we experience qualities that can vary independently (modes), and we experience comparisons between things (relations). The chapter thus provides the map for the chapters that follow: Book II will now proceed to examine each of these three kinds of complex ideas in order, beginning with modes, then moving to substances, then relations.

Book II: Of Ideas — Chapter XIII: Of Simple Modes, and First, of the Simple Modes of Simple Ideas of Sensation

We now enter Locke’s detailed treatment of modes, beginning with simple modes — variations or different degrees of the same simple idea, without mixing it with other simple ideas. This chapter takes up the simple modes of space, and it is the first extended application of the method announced in Chapter XII.

Locke begins by defining simple modes as “those variations of the same simple idea, which the mind either finds in objects, or is able to make within itself, without the mixture of any other idea.” He gives the example of a dozen: we have the simple idea of unity, and by repeating it we produce the complex ideas of a dozen, a score, or a million — all simple modes of unity. Similarly, duration and number are simple modes. The chapter then turns specifically to the simple modes of space, which Locke explores through a series of related ideas: distance, capacity (or extension), and figure. He traces how the mind, beginning with the simple idea of space gained through sight and touch, can form the idea of a foot or a mile by repeating a standard measure, and then can form the idea of immensity by continuing this repetition without stopping.

Locke then introduces a crucial distinction between extension and body. The idea of pure space is distinct from the idea of body because space has no solidity — it is “a capacity or possibility of receiving body.” Solidity, as established in Chapter IV, is the idea of resistance that bodies give to other bodies entering their place. Pure space has extension but not solidity; body has both. This distinction is necessary, Locke argues, to avoid confusing the idea of space with the idea of the matter that fills it. He then explores how the idea of figure arises: “when we see a piece of gold,” he writes, “the figure is the termination of its extension.” By considering the boundaries of extension, the mind forms ideas of particular figures — circle, square, triangle — and can then combine these into infinitely complex figures. This chapter thus shows how the mind, working entirely from the simple idea of space, can generate the whole science of geometry.

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

Locke now turns to another primary simple idea of both sensation and reflection: duration. This chapter is one of the most philosophically significant in the entire Essay, as it addresses the origin of our idea of time. Duration is a simple mode, like space, but it comes primarily from reflection — from the mind’s observation of its own train of ideas. Locke’s account of time is thoroughly empirical, grounding even this abstract concept in experience.

Locke begins by observing that we get the idea of succession from the constant train of ideas that passes through our minds while awake. “It is evident to any one who will but observe what passes in his own mind,” he writes, “that there is a train of ideas which constantly succeed one another in his understanding, as long as he is awake.” Reflection on this succession gives us the idea of duration. We then measure duration by applying some regular periodical measure — the motion of the sun, the swing of a pendulum, the revolution of the earth — and we call these measured periods time. But Locke is careful to distinguish duration itself from any particular measure of it: duration is the uninterrupted flux of the mind’s ideas, while time is the application of a standard measure to that flux.

Locke then explores several puzzles about duration. He asks whether the mind can have an idea of eternity — and answers that it can, by repeating the idea of any finite duration (a year, a day, an hour) infinitely, just as we formed the idea of immensity by repeating the idea of space. He also examines the relationship between duration and expansion (space). Both are simple modes capable of infinite extension in the imagination, but expansion is a capacity for receiving bodies, while duration is the continued flux of existence. The chapter concludes with a striking observation: our idea of duration is proportional to the number of ideas that pass through the mind in a given period. A mind full of thoughts experiences time as longer than a sluggish mind; this is why children, whose minds are constantly receiving new impressions, seem to experience time as passing more slowly than adults. This is not merely a psychological observation — it follows from Locke’s definition of duration as the reflection on the train of ideas.

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

In this short but important chapter, Locke compares duration and expansion (space) directly, showing both their similarities and their essential differences. This chapter consolidates the arguments of the two previous chapters and draws some significant metaphysical conclusions.

Locke identifies several points of likeness between duration and expansion. Both are simple ideas that the mind can extend infinitely by repetition. Both are continuous quantities with parts that can be measured. Both are “affections of finite beings” — that is, they are ways we conceive of things having existence in time and place. But Locke is more interested in their differences. The crucial difference is that the parts of expansion are “inseparable, immovable, and fixed” — space stays where it is — while the parts of duration are “in a perpetual flux, and succeed one another.” A moment of time passes and cannot be recovered; a portion of space remains where it was. This leads to a striking conclusion: “Duration and expansion, though they are both considered as having parts, yet their parts are not at all alike.”

Locke then addresses a question that arises naturally: if we measure duration by motion (the sun’s revolution), and we measure expansion by motion (how far something travels in a given time), are time and space the same thing? No, Locke answers. Motion measures both, but it measures them as different things: “motion does not measure duration any otherwise than as it brings the ideas of successive ideas into the mind.” The motion of the sun makes us aware of the passage of time, but time itself is not motion; it is the reflection on succession. Similarly, an hour of pain is different from an hour of pleasure, even though motion measures both the same way. The chapter thus reinforces Locke’s fundamental thesis that our ideas of time and space are constructed from the simple ideas of reflection (succession) and sensation (extension), and that confusing the two leads to philosophical error.

Book II: Of Ideas — Chapter XVI: Of Number

Locke now examines the simple mode of number, which he considers the simplest and most universal of all ideas. Number, he argues, is the idea that the mind applies most readily and that is most “abstract, general, and universal” — every thing that exists can be conceived as one or more. This chapter is relatively brief but philosophically important for its account of how the mind forms mathematical concepts.

Locke begins with the observation that number is “the most simple and most universal idea we have” because it is the idea of unity repeated. Every object of sensation or reflection gives us the idea of one (one man, one thought, one body), and the mind can repeat this idea without limit. Number is a simple mode because it arises from the repetition of the simplest of all ideas — unity — without mixing it with any other idea. Locke notes that number is the only idea that can be called “clear and distinct” in the highest degree: “the ideas of a line, a circle, or a triangle, are not so clear and distinct as the ideas of two, three, and four.”

Locke then makes a crucial point about language and mathematics. He observes that our ability to use numbers for calculation depends entirely on our ability to give them names and marks (numerals). Without fixed names for numbers, the mind could not keep track of large combinations; “without such names or marks, we can hardly well make use of numbers in reckoning.” This is why children must learn the names of numbers before they can count. Locke also notes that different cultures have developed different number systems, and that the invention of arithmetic is one of the greatest achievements of human reason. The philosophical importance of number, for Locke, lies in its demonstration of how the mind can build the most abstract and universal ideas from the simplest of experiences — the idea of one thing — and then, through the power of naming and repeating, construct the entire edifice of mathematics. Number also serves as the paradigm for all other simple modes: it shows how repetition of a single simple idea produces complex ideas of unlimited variety.

Book II: Of Ideas — Chapter XVII: Of Infinity

We are moving from the simple modes of number, duration, and expansion into an examination of the idea of infinity, which Locke derives from the operation of the mind upon these prior ideas.

The chapter opens with a careful distinction: “infinity” is properly an idea of space, duration, or number, not a positive idea of an infinite being or quality. Locke argues that what we call infinity is actually the idea of “supposed endless progression.” We arrive at it by observing that the mind can repeat any finite idea—of a foot, a minute, or the number one—without ever being forced to stop. As Locke writes: “Whatsoever positive ideas we have in our minds of any space, duration, or number, let them be never so great, they are still 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 infinity.” This is the crucial point: our idea of the infinite is not a positive grasp of something unlimited, but the negative idea of a limit we cannot reach. We do not positively comprehend infinite space; we only know that wherever we stop, we can always go further.

Locke then applies this analysis to the three realms. In number, we have the clearest and most positive idea of infinity, because numbers can always be added to without contradiction—we can always conceive of one more number. In duration, we can conceive of an endless succession of moments stretching backward and forward, though Locke notes this is harder because we cannot actually experience endless time. In expansion (space), we can similarly conceive of boundless extension, but here the idea becomes most troublesome, because we cannot help imagining space as something real with parts beyond parts. The difficulty with infinity in space is that we are tempted to treat it as a positive, actually existing infinite magnitude—but Locke insists that we have no such positive idea. The idea of infinity is always relative: it is the idea of “not being able to be ended.”

The clarifying point for Locke’s epistemology is that infinity does not come from sensation or reflection directly. It is constructed by the mind’s own operation of repetition without cessation. This means the idea of infinity is a mode, not a substance or a quality of things in themselves. Locke warns against pretending we have a clear positive idea of the infinite, as when philosophers argue about “infinite space” as if it were a thing we comprehend. For him, we must accept that our idea of infinity is “very obscure and imperfect” precisely because it is negative—it tells us what cannot be done (find an end) rather than what positively is. This chapter therefore serves as a critique of metaphysical overreach: we can use the idea of infinity in mathematics and in reasoning about God’s eternity, but we must not mistake it for a clear and distinct perception of something actually infinite.

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

We are now at a short transitional chapter where Locke catalogs the remaining simple modes not covered by the major categories of space, duration, number, and infinity.

The chapter addresses two principal groups: the simple modes of motion and thinking. For motion, Locke notes that the simple modes consist entirely of degrees of velocity—faster and slower. The idea of motion itself comes from sensation (seeing or feeling things move), and then the mind can compare different speeds, producing modes like “swift” and “slow.” These are simple modes because they involve only the repetition and variation of one simple idea: motion. For thinking, Locke offers a more elaborate treatment. The simple idea of thinking comes from reflection, but the modes of thinking are extraordinarily diverse. They include perception, remembrance, contemplation, reasoning, judging, willing, loving, hating, and all other acts of the mind. Each of these is a different mode of the same fundamental operation—thinking—just as different shapes are modes of extension.

Locke makes an important observation about why the modes of thinking are more difficult to enumerate than the modes of motion or space. The reason is that we have names for only some of them, and the boundaries between them are often obscure. We can easily tell a square from a circle, but the difference between “judging” and “opining” is harder to fix. As Locke says: “The ideas that are in the mind are what everyone can best know in himself; and it is from the observation of his own mind that he must learn what the several modes of thinking are.” This chapter thus serves as a reminder that the simple modes are not all equally well-defined by language. Some, like motion and extension, produce clear and distinct modes with stable names; others, like the varieties of thought, blur into one another. The point for Locke’s system is that the mind’s own operations—the ideas of reflection—produce their own family of simple modes, and these modes are just as real and just as much products of experience as those derived from sensation.

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

We deepen the previous chapter’s theme by focusing entirely on the modes of thinking, with special attention to the distinction between active and passive states of mind.

Locke begins by distinguishing sensation from thinking proper. Sensation is the simple perception of an external object through the senses; thinking in its fullest sense is the mind’s voluntary employment of its own ideas. He then catalogues the main modes of thinking. Perception is the simplest and most passive: it is the actual reception of an idea into the mind. Memory is the retention and revival of past perceptions. Contemplation is the mind’s steady application to a present idea. Attention is the fixing of the mind on one idea to the exclusion of others. Study is intense and sustained contemplation directed toward learning. Reasoning is the perception of connections between ideas. Judgment is the perception of agreement or disagreement between ideas. Willing is the power of the mind to direct its own operations or to prefer one action over another.

Locke then introduces a critical distinction that will become central to later debates: the difference between the understanding and the will. The understanding is the power of perception—receiving and comparing ideas. The will is the power of volition—preferring and choosing. Both are modes of thinking, but they are distinct operations. A person can perceive something clearly and yet not will it; they can will something without fully understanding it. Locke writes: “The will and the understanding may be distinguished; they are two distinct powers in the mind, as far as we can conceive of them.” However, he immediately cautions that we should not reify these powers—they are not separate agents or faculties within us, but simply different ways the same mind operates. The distinction is practical, not metaphysical.

This chapter is important because it lays the groundwork for Locke’s later discussion of human freedom and the will in Book II, Chapter XXI. It shows that the modes of thinking are not a homogenous mass but contain active and passive elements, voluntary and involuntary operations. The mind is not merely a passive receiver of sensations; it actively compares, judges, and wills. Yet all of these operations are still modes—variations on the original simple idea of thinking derived from reflection. The clarity of this analysis depends on the reader’s willingness to observe their own mind, for only introspection can reveal the precise difference between, say, remembering and contemplating.

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

We now turn to an entirely different class of simple modes: those of pleasure and pain, which Locke treats as fundamental accompaniments to almost every other idea.

Locke announces at the start that pleasure and pain are “simple ideas” that come into the mind by both sensation and reflection. When we feel heat, if it is moderate, we experience pleasure; if excessive, pain. When we contemplate truth or virtue, we experience intellectual pleasure; when we think of evil or falsehood, intellectual pain. These ideas are “annexed” to almost all other ideas by the Author of Nature, as a kind of moral and practical guide. Locke writes: “Pleasure and pain are simple ideas, which we receive from the objects of sensation and reflection; for sensation and reflection, as we have shown, are the only ways that ideas come into the mind.” He then distinguishes the modes: joy is pleasure arising from the thought of the present or certain possession of a good; delight is pleasure in the actual enjoyment of a present good; satisfaction is the mind’s rest in the possession of good. On the negative side, sorrow is pain at the loss of a good; hope is pleasure from the expectation of future good; fear is pain from the expectation of future evil; despair is the complete absence of hope; anger is displeasure at a perceived injury; and so on through a catalogue of the passions.

Locke’s treatment is notable for its grounding of complex emotional states in the simple ideas of pleasure and pain. All the passions—love, hatred, desire, joy, sorrow, hope, fear, anger, envy, shame—are modes of these two fundamental feelings. Love, for example, is defined as “a pleasure in the presence of the idea of a thing”; hatred as “a pain in the presence of the idea of a thing.” This reduction of the emotional life to two simple ideas is characteristic of Locke’s empiricism: he wants to show that even our most complex passions are built from the simplest experiential materials. The significance of the chapter lies in its moral implications: if pleasure and pain are the fundamental motivators of human action, then morality itself must be understood in terms of what brings the greatest pleasure and least pain. This prepares the ground for the hedonistic psychology that will appear later in the *Essay*. It also demonstrates that the mind’s inner world of feeling is as susceptible to empirical analysis as the outer world of sensation.

Book II: Of Ideas — Chapter XXI: Of Power

This is one of the longest and most important chapters in the entire *Essay*, where Locke addresses the idea of power and, through it, the nature of human freedom, the will, and moral responsibility.

Locke begins by stating that the idea of power comes from two sources: from sensation, by observing the changes in external objects (fire melts wax, the sun whitens wax), and from reflection, by observing the mind’s own ability to direct its thoughts and actions. The idea of power is therefore a complex idea derived from both sensation and reflection, and it has two fundamental aspects: active power (the ability to produce change) and passive power (the capacity to receive change). Locke argues that our clearest idea of active power comes from reflection—specifically, from our experience of willing. When we will to move our arm, and it moves, we have direct experience of active power. From external objects, we only ever see change, not the active power that produces it—we see fire melt wax, but we do not see the “power” itself. Locke writes: “I think we have from reflection the idea of power, as far as we ourselves are agents; and from sensation we have the idea of power, as far as we find ourselves acted upon.”

Locke then turns to the will and volition. The will is defined as “a power of the mind to direct its own operations to the attainment of some end.” Volition is the actual exercise of that power. He introduces a crucial distinction between voluntary actions (those we will) and involuntary actions (those we do not will). The fundamental question of freedom is whether the will itself is free, or whether it is determined by something else. Locke argues that the phrase “freedom of the will” is a category mistake. The will is a power, and powers are not free or unfree—only agents are free. A man is free when he has the power to act or not act according to his own choice. Locke writes: “A man is at liberty to lift up his hand or let it fall; but the will itself is determined by something in the mind.” The real question is what determines the will.

The answer Locke gives is uneasiness. The will is determined by the most pressing uneasiness a person feels. Desire is a form of uneasiness—the absence of some good creates a felt lack, and the mind is moved to relieve that uneasiness. This is not a mechanical determinism, because the mind can suspend its desires, consider the consequences, and choose among competing goods. This capacity for suspension is the foundation of human freedom: we can pause between the perception of a desire and the act of willing, and in that pause we can judge whether the good is truly good. Locke argues that true freedom consists not in an undetermined will, but in the power to suspend our desires and deliberate rationally. The chapter concludes with a lengthy discussion of how this relates to moral responsibility: we are accountable for our actions because we have the power to examine our desires and choose accordingly.

This chapter is philosophically dense and represents Locke’s mature answer to the problem of free will. He rejects both the theological determinist (who says God predestines all actions) and the radical libertarian (who says the will chooses without any cause). Instead, he offers a compatibilist view: the will is determined by desire, but we can shape our desires through rational deliberation. The idea of power is thus central to Locke’s entire moral and psychological system—it connects the simple ideas of sensation and reflection to the highest questions of human agency and responsibility.

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

We now move from simple modes to mixed modes, which Locke defines as complex ideas composed of several simple ideas of different kinds, combined by the mind into a single unified idea.

Locke begins by distinguishing mixed modes from simple modes. Simple modes are variations of a single simple idea (like a dozen, which repeats the idea of one). Mixed modes combine ideas of different sorts. For example, the idea of drunkenness combines the simple ideas of drinking, excess, and the alteration of mental state. The idea of murder combines killing, intention, malice, and the killing of a human being. These mixed modes are not given by nature—they are fabrications of the mind that we voluntarily combine and name. Locke emphasizes that unlike substances (which are supposed to have a real essence in nature), mixed modes have no standard in reality beyond the combinations we choose to make. He writes: “The mind, in respect of its simple ideas, is wholly passive; but in respect of its complex ideas, it is active; and many of them are of its own making.”

Locke then explores how mixed modes are formed. There are three ways: first, by combining several simple ideas of different kinds; second, by comparing ideas to find their relations; third, by abstracting from particular circumstances to form general ideas. Mixed modes are particularly important for moral and legal language, because concepts like “virtue,” “vice,” “justice,” and “crime” are mixed modes—they are combinations of simple ideas that we have agreed to name. There is no “essence” of justice in nature; justice is whatever collection of simple ideas we have decided to call by that name. This means that moral terms are, in an important sense, of our own making. The clarity and precision of moral discourse depends on how carefully we define our mixed modes.

The significance of this chapter is immense for Locke’s theory of language and morality. It explains why moral disputes are often so difficult to resolve: different people may be using the same word (“justice”) to refer to different collections of simple ideas. Once we realize that mixed modes are human constructions, we can see that moral concepts are not discovered in nature but invented by the mind for purposes of social coordination. This does not make morality arbitrary, because the simple ideas that compose mixed modes come from experience—but it does mean that the boundaries of moral concepts are fixed by human agreement, not by the nature of things. This chapter therefore prepares the ground for Locke’s later discussions of moral philosophy and the social contract.

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

We now arrive at a major turning point: Locke’s treatment of substances, the most fundamental kind of complex idea and the basis for our understanding of the external world.

Locke begins by noting that the mind receives many simple ideas from sensation and reflection. It notices that certain simple ideas constantly go together—for example, the color, weight, hardness, and malleability of gold appear together. The mind, supposing that there is some substratum in which these qualities inhere, gives this combination a name (“gold”) and calls it a substance. But Locke immediately warns that we have no clear positive idea of what this substratum is. The idea of substance in general is “nothing but the supposed, but unknown, support of those qualities we find existing.” Locke writes: “The idea of substance is nothing but a supposition of we know not what support of such qualities as are capable of producing simple ideas in us.” We are like someone who sees a clock’s hands moving but has no idea of the internal mechanism—we know there must be something underlying the appearances, but we do not know what it is.

Locke then distinguishes between three kinds of ideas of substances: God, finite intelligences (angels and spirits), and bodies. Our idea of bodies comes from sensation: we combine simple ideas like extension, solidity, motion, color, taste, etc., and suppose a substratum that unites them. Our idea of spirits comes from reflection: we combine simple ideas like thinking, willing, and power, and suppose a substratum for them. Our idea of God is derived by enlarging the ideas we have of our own minds—we ascribe infinity, eternity, omniscience, and omnipotence to a substance without body. For each of these, the substratum itself remains unknown. What we know are the secondary qualities (colors, sounds, tastes) and primary qualities (solidity, extension, figure, motion) that appear to us, but the real essence—the internal constitution that produces these qualities—is beyond our knowledge.

This chapter is the foundation of Locke’s representational realism. We do not know things as they are in themselves; we know only the ideas they produce in us. The idea of substance is a necessary postulate of thought—we cannot help supposing that there is something underlying the qualities—but it is an obscure and relative idea. The consequence for Locke’s epistemology is that human knowledge of the physical world is limited to the observable properties and powers of bodies. Science (what Locke calls “natural philosophy”) can investigate these properties, but it can never penetrate to the real essences of things. This humility about what we can know will shape the rest of the *Essay*.

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

We are still within the domain of substances, but now Locke treats collective ideas—complex ideas made by putting together several individual substances into one idea.

Locke observes that the mind has the power to consider several distinct substances together as one idea. For example, the idea of an army combines many individual men; the idea of a constellation combines many stars; the idea of a flock combines many sheep. These collective ideas are complex ideas of substances, but they are not ideas of a single substance—they are ideas of a collection. Locke writes: “These are collections of substances, such as the idea of a flock of sheep, a fleet of ships, an army of men; which are made up of many particular substances, considered as having some relation to each other.” The key is that the relation—spatial proximity, common purpose, or some other tie—is what unifies the collection in the mind’s view.

Locke makes an important point about the names of these collective ideas. Words like “army,” “fleet,” and “flock” are general names that denote whole collections, not individual substances. They function like other general terms in that they allow us to think and speak about groups as if they were single things. But Locke warns that we should not reify these collections: an army is not a substance itself but a collective mode. The philosophical significance is that this shows how the mind can combine even the most complex ideas of substances into higher-order unities. It demonstrates the constructive power of the understanding, which can take the raw materials of experience (simple ideas) and build ever more complex structures. Collective ideas also have practical importance: much of social and political life depends on treating groups (armies, cities, nations) as unified agents, even though they are really collections of individuals.

Book II: Of Ideas — Chapter XXV: Of Relation

We now enter a new domain: relation, which Locke treats as a distinct kind of complex idea formed by comparing two or more ideas.

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 to each other, it forms an idea of relation. The things compared are called the subject and the related term. For example, in “Cain is the father of Abel,” Cain is the subject and Abel is the related term. The relation “father” connects them. Locke emphasizes that the relation itself is not a quality of either thing but a third idea produced by the mind’s act of comparison. He writes: “The nature of relation consists in the comparing of one idea to another; and it is from that comparison that the idea of relation is derived.” Relations are therefore not in the things themselves but are the work of the mind.

Locke then classifies relations into several kinds. There are relations based on cause and effect: the idea of a cause (that which produces any simple or complex idea) and an effect (the idea produced). There are relations of time: before and after, older and younger. There are relations of place: farther and nearer, above and below. There are proportional relations, such as in mathematics (greater and less, double and half). And there are moral relations, which compare human actions to a law or rule. For each kind of relation, Locke notes that the mind creates the comparison and then names the resulting idea. The relation itself is “extraneous” to the things compared: a father is a man first, and only a father in relation to his child.

The importance of this chapter is that it shows how the mind’s active power of comparison generates an entirely new class of ideas—ideas that are not found in sensation or reflection directly. Relations are the mind’s own contribution to knowledge. This anticipates Kant’s later emphasis on the active role of the understanding, though Locke is more modest: for Locke, relations are real ideas even if they are not qualities of things in themselves. The practical consequence is that much of our language and reasoning depends on relations—we cannot think about society, morality, or mathematics without comparing ideas. Understanding how relations work helps us avoid confusion: when we say “Cain is a father,” we should remember that “father” is not a property of Cain alone but a relation to Abel.

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

We deepen the previous chapter by focusing on the most fundamental of all relations: cause and effect.

Locke states that we observe in experience that one thing regularly produces a change in another. Fire produces heat; the sun melts wax; a moving billiard ball sets another in motion. From such observations, we form the idea of causality: the relation between that which produces (the cause) and that which is produced (the effect). Locke defines cause as “that which produces any simple or complex idea, considered as producing it,” and effect as “that which is produced.” He emphasizes that we have no direct perception of the “power” that connects cause and effect—we only see the constant conjunction. The idea of necessary connection is not given in sensation; it is something we project onto the observed sequence.

Locke then extends this analysis to other relations. Identity and diversity are relations: we compare an object to itself at different times and judge whether it is the same or different. Proportional relations are those of quantity, such as larger, smaller, equal, double. Natural relations are those based on blood or kinship, such as father, mother, brother. Instituted relations are those created by human agreement, such as husband and wife, master and servant. Moral relations are those that compare human actions to a rule or law. For each type, Locke notes that the relation is a product of the mind’s comparison and that understanding the relation requires understanding the terms being compared.

This chapter is crucial because it establishes the empirical basis for causation—arguably the most important concept in natural science. Locke’s analysis is a precursor to Hume’s more radical skepticism about causation. For Locke, cause and effect is a real relation that we observe in experience, but we do not have a clear idea of the necessary connection between them. This means that our knowledge of causal laws is probabilistic rather than certain. The chapter also reinforces Locke’s general point that relations are of the mind’s making: the world gives us sequences of events; the mind adds the idea of “cause” through comparison. This insight will have profound implications for Locke’s epistemology: if even causation is a relative idea, then our claims to know the necessary structure of reality are more limited than we might think.

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

This is one of the most famous chapters in the entire *Essay*, where Locke introduces his theory of personal identity.

Locke begins by noting that the idea of identity arises from comparing a thing with itself at different times. A thing is the same as itself if it has the same beginning of existence; it differs from another if it began at a different time. The crucial question is what makes a person the same person over time. Locke distinguishes three kinds of identity: the identity of inanimate objects (a mass of matter remains the same if it is the same particles), the identity of living beings (a plant or animal remains the same if it has the same life and organization, even as its particles change), and the identity of persons. A person, for Locke, is “a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing, in different times and places.” The identity of a person is not the identity of the same substance (the same soul or the same body); it is the identity of the same consciousness.

Locke argues that personal identity consists in consciousness: a person is the same person as long as they can remember past experiences as their own. If a prince’s consciousness enters the body of a cobbler, the person is the prince, not the cobbler. If a man loses all memory of his past actions, he is not the same person who committed them. Locke writes: “As far as this consciousness can be extended backwards to any past action or thought, so far reaches the identity of that person; it is the same self now as it was then; and it is by the same self with this present one that now reflects on it, that that action was done.” This means that memory is the criterion of personal identity. The body may change, the soul may change (if the soul is a thinking substance), but as long as consciousness is continuous and memory connects past and present, it is the same person.

This chapter has profound implications for morality, law, and theology. If personal identity is based on consciousness, then a person cannot be held accountable for actions they do not remember. A drunk man who commits a crime while drunk but remembers nothing of it in the morning is not, strictly speaking, the same person who committed the crime—though Locke notes that society may still punish him for practical reasons. Similarly, the doctrine of the Resurrection is reinterpreted: what matters is not that the same body is raised, but that the same consciousness continues. Locke’s theory of personal identity is one of the most influential ideas in the *Essay*, shaping subsequent debates in philosophy about the self, memory, and moral responsibility. It shows how the simple ideas of reflection (thinking, memory, consciousness) can be used to analyze even the most intimate and seemingly indestructible concept: the self.

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

We now examine a final set of relations, particularly moral relations, before moving on to the third book of the *Essay*.

Locke divides relations into three categories based on what we compare. The first is the relation of cause and effect, already covered. The second is the relation of proportion, covering mathematical relations like equality and inequality. The third is the relation of moral good and evil, which compares human actions to a law. Locke argues that all moral relations fall under three kinds of law: the divine law (God’s law, discovered by reason or revelation), the civil law (the law of the state, enforced by the magistrate), and the law of opinion or reputation (the social norms of a community, enforced by praise and blame). These three laws provide the standards against which we judge actions as virtuous or vicious, just or unjust, praiseworthy or blameworthy.

Locke explains that the divine law is the most important because it carries eternal rewards and punishments. The civil law governs what is legal or illegal in a society. The law of opinion governs what is socially acceptable—what we call “virtue” and “vice” in everyday life. Locke observes that in most societies, what is called virtue is what receives public praise, and vice is what receives public blame. This does not make morality merely relative, because the divine law provides an objective standard. But it does explain why moral terms can vary across cultures: different communities have different laws of opinion. Locke writes: “The measure of what is every one’s duty is the law; and that is the rule of virtue and vice.” This chapter thus connects Locke’s theory of relations to his moral philosophy, showing how the relation between an action and a law determines its moral character.

The significance of this chapter is that it gives Locke a framework for analyzing moral language without invoking innate ideas. Moral terms are relational: they compare actions to laws. The laws themselves (divine, civil, social) are given by experience—either by revelation, by political institutions, or by social custom. This makes morality a matter of relations rather than of innate principles. The chapter also prepares the ground for Locke’s later political writings, particularly his defense of religious toleration: if the civil law and the divine law are distinct, then the state should not enforce religious conformity.

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

We are now near the end of Book II, and Locke pauses to reflect on the quality of our ideas, introducing distinctions that are crucial for evaluating the reliability of knowledge.

Locke defines a clear idea as one that is “such as the object that causes it would produce in a well-disposed organ” — that is, an idea that presents itself vividly to the mind, as a well-lit object appears to the eye. An obscure idea is the opposite: faint, dim, like an object seen in poor light. A distinct idea is one where the mind can perceive its component parts and distinguish it from all other ideas. A confused idea is one where the parts are jumbled or the boundaries are unclear. Locke uses the analogy of vision: a clear idea is like seeing an object in bright daylight; a distinct idea is like seeing all its parts and edges clearly separated from the background. He writes: “A clear idea is that whereof the mind has such a full and evident perception as it does receive from an external object operating on a well-disposed organ. A distinct idea is that wherein the mind perceives the difference from all other ideas.”

Locke then applies these distinctions to different kinds of ideas. Simple ideas are generally the clearest and most distinct: we know exactly what we mean by “white” or “sweet.” Complex ideas of substances are often obscure and confused because we do not know the real essence of gold or water—we only have a collection of observable qualities. Mixed modes, on the other hand, can be made perfectly distinct because we define them ourselves: we can define exactly what we mean by “murder” or “gratitude.” The confusion of ideas usually arises from the use of unclear language: we use words without having a clear and distinct idea in our minds.

This chapter functions as a diagnostic tool for the rest of the *Essay*. It gives Locke a way to criticize philosophers who use words without clear ideas. The aim of the understanding should be to have clear and distinct ideas, especially in moral and metaphysical disputes. The chapter also shows Locke’s debt to Descartes’s criterion of clear and distinct perception, though Locke applies it more cautiously: we can have clear ideas of secondary qualities, but these are not clear ideas of the things themselves. The point is to make us conscious of the limits of our understanding: we should not claim to have knowledge where our ideas are obscure and confused.

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

We continue Locke’s evaluation of ideas with the distinction between real ideas (those that have a foundation in nature) and fantastical ideas (those that are mere inventions of the mind).

Locke defines real ideas as those that “have a foundation in nature; such as have a conformity with the real being and existence of things, or with their archetypes.” Fantastical ideas are those that “have no foundation in nature, nor any conformity with that reality of being to which they are tacitly referred.” Simple ideas are all real because they are produced in us by the operation of external objects or by the mind’s own operations. Even the simple idea of “red” is real because it is produced by a real power in the object, even if the red is not a quality of the object itself. Complex ideas of substances are real if they are copies of real combinations in nature—the idea of gold is real if there actually exist objects with that combination of qualities. But complex ideas that we invent without corresponding to anything in nature—like a centaur (combining man and horse) or a golden mountain—are fantastical.

Locke notes that mixed modes and relations are different: they are real in the sense that they exist as ideas in the mind, but they do not claim to represent anything in nature. The idea of “murder” is real as a mental construct, but it does not correspond to an external object—it is a combination we have made. The distinction matters because it tells us which ideas we can rely on for knowledge of the external world. Real ideas of substances are our only window onto the physical world; fantastical ideas are products of the imagination and should not be mistaken for reality. Locke writes: “Our simple ideas are all real, all conformable to the reality of things; but our complex ideas are not all so; for we may join the idea of a horse with that of a man, and make a centaur, which is a fantastical idea of a substance.”

The philosophical takeaway is that we must be careful not to treat our invented combinations as if they were discovered in nature. Many philosophical errors, in Locke’s view, come from treating fantastical ideas as real—for example, treating the idea of “substance” as if it were a clear positive idea of a thing, rather than an obscure supposition. The chapter reinforces Locke’s empiricism: the only sure foundation for knowledge is the simple ideas given by sensation and reflection. Complex ideas are trustworthy only when they faithfully copy the combinations found in nature.

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

We now consider whether our ideas are adequate (completely representing their archetype) or inadequate (partial or imperfect representations).

Locke defines an adequate idea as one that “perfectly represents its archetype” — that is, it contains all the real qualities that belong to the thing it represents. An inadequate idea is one that represents only part of the archetype. Simple ideas are all adequate because they are exactly what they are: the idea of “white” perfectly represents the power in the object to produce that sensation in us. There is nothing missing from the idea of “white” — it is the complete idea of that sensation. For mixed modes, the ideas are also adequate because the archetype is the idea itself: we define what “murder” means, so our idea cannot be inadequate to its own definition. For substances, however, our ideas are radically inadequate. We have only a few of the qualities that exist in the substance; we do not know the real essence; we do not know how the qualities are connected. The idea of gold includes yellowness, weight, malleability, and solubility in aqua regia — but there are probably many more qualities we have not discovered, and we do not know why these qualities cohere.

Locke writes: “Our complex ideas of substances are all inadequate; because they are not copies of the real essences of things, but only collections of those simple ideas that are observed to co-exist together.” The consequence is that our knowledge of the physical world is always partial and subject to revision. We can discover new qualities of gold, and our idea of gold will then expand. This is not a defect of the understanding but a condition of its operation: we are finite beings with limited senses. The distinction between adequate and inadequate ideas therefore reinforces Locke’s epistemic humility: we should not claim certainty about the natures of substances when our ideas of them are inevitably incomplete.

This chapter also has implications for the limits of science. If our ideas of substances are always inadequate, then we can never have a complete science of nature. We can collect observations and form hypotheses, but we cannot penetrate to the real essences of things. This is a critique of the Aristotelian tradition, which claimed to know the essential natures of substances. For Locke, the real essences of substances (their internal constitutions) are unknown and probably unknowable to us.

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

This is the final chapter of Book II proper (before the concluding summary chapter), and Locke addresses whether ideas themselves can be called true or false.

Locke begins with a caution: strictly speaking, ideas are not true or false. Truth and falsehood belong to propositions (statements that affirm or deny something), not to single ideas. An idea is simply a perception in the mind—it is what it is. But ideas can be called true or false in a secondary sense, when they are referred to something outside themselves. Locke identifies four kinds of reference: (1) when an idea is referred to a real thing in nature (as the idea of gold is supposed to correspond to actual gold); (2) when an idea is referred to the idea of a substance that other people call by the same name (as when my idea of “man” is supposed to match the common idea of “man”); (3) when an idea is referred to a real essence (as when philosophers talk about the essence of a species); and (4) when an idea is referred to the existence of something.

In each case, an idea is “true” if it conforms to its archetype; “false” if it does not. Thus, the idea of a centaur is not false in itself—it is a real idea in the mind. But if someone believes that a centaur actually exists in nature, then their idea is false in that reference. Simple ideas are always true because they are produced directly by the operation of objects—they cannot misrepresent because they are the immediate effect. Complex ideas of mixed modes are also always true because the archetype is our own definition. Only complex ideas of substances can be false, because we may wrongly believe that our collection of simple ideas corresponds to a real thing in nature. Locke writes: “Our complex ideas of mixed modes, being voluntary collections of simple ideas, which the mind puts together, without reference to any archetype, cannot be false, because they have no standard with which to compare them.”

This chapter serves as a fitting conclusion to Book II’s treatment of ideas. It shows that error does not arise from the ideas themselves—they are what they are—but from the judgments we make about them, particularly when we assume that our complex ideas of substances correspond perfectly to external reality. The chapter thus prepares the ground for Book III, which will examine language and the way words can lead us into error by making us think our ideas are more adequate than they are.