The Essay opens with a scene. Five or six friends meet in Locke's rooms, argue about morality and revealed religion, get nowhere, and realize they are stuck because they have no account of what the human mind can do in the first place. Locke wrote a page on it that evening. Nineteen years later it was four books.

That origin is the shape of the whole thing. It is not a system of the world. It is a survey of the instrument, conducted before any claim about the world is allowed through.

What reading it is like

Four books, around 270,000 words, in plain seventeenth-century English that is easier than almost anything else from the period. Locke writes as though he is talking to you, which is pleasant, and he repeats himself, which is not.

Book II is the one. If you read one thing, read Book II, and within it the chapters on qualities (viii), on substance (xxiii) and on identity (xxvii). Book I is a long polemic against a position nobody now holds. Book III, on language, is better than its reputation and is usually skipped. Book IV is the payoff and is hardest.

The five editions matter more than usual: the chapter on personal identity was not in the first edition at all, and the one on the association of ideas arrived in the fourth. What is quoted as the Essay is a text Locke kept arguing with for eleven years.

Book I: nothing is written on it yet

The target is innate ideas — the doctrine that certain principles are stamped on the mind at birth. Locke's argument is that if they were, everyone would have them, and children and what he calls the idiot and the illiterate do not.

As philosophy this is weak, and it has been answered since Leibniz: an innate capacity need not be a conscious possession, and nobody serious claimed it was. As rhetoric it did its job completely. After Book I the burden of proof changed sides, and anyone wanting to claim that something was known prior to experience had to say how.

Book II: where everything comes from

The positive account, and the part that founded a tradition.

The mind at birth is white paper, void of all characters. Everything on it arrives from one of two places: sensation, the outward senses reporting the world, and reflection, the mind noticing its own operations — perceiving, doubting, willing, remembering. There is no third source. Ideas are either simple, received whole and passively, or complex, built by the mind out of simple ones by combining, comparing and abstracting.

Three chapters in Book II have outlived the rest.

Qualities (II.viii). Some properties are really in the object — solidity, extension, shape, motion, number. Others are only powers in the object to produce ideas in us — color, sound, taste, smell. The snow is not white in the way it is cold; the whiteness is something that happens where the light meets an eye. The distinction is older than Locke, and his version is the one the modern argument uses.

Substance (II.xxiii). If all we have are ideas of qualities, what are the qualities qualities of? Locke says we suppose some substratum underneath and that we have no idea of it whatever — a something, I know not what. He is not denying it exists. He is saying empiricism cannot reach it, and leaving the admission sitting in the middle of his own book, which is intellectually honest and, as Berkeley immediately noticed, a very large hole. See substance.

Identity (II.xxvii). What makes you the same person as the child in the photograph is not the same body and not the same soul, but the same consciousness — where your memory reaches, you reach. The cases are the famous part: the prince whose memories wake up in a cobbler's body is the prince; the man who genuinely cannot remember the crime is, in the sense that matters for punishment, not the man who did it. Two and a half centuries of argument have started here. See personal identity.

Book III: words

An entire book on language, which was unusual, and it contains the best-argued nominalism of the century. General terms do not name real kinds in nature. They name nominal essences — the abstract ideas we have assembled and agreed to call gold or horse. The real essence, the inner constitution that actually makes gold behave as it does, is not something we have access to, so it is not what our word means.

Most of the book is about how language goes wrong, and the complaints are recognizable: words used without clear ideas behind them, words whose meaning shifts mid-argument, disputes that are about vocabulary and are conducted as though they were about the world. See nominalism.

Book IV: what is left

Knowledge, for Locke, is the perception of agreement or disagreement between ideas, and it comes in three grades. Intuitive — seen immediately, as that white is not black. Demonstrative — seen through a chain, as in a proof. Sensitive — that particular things exist outside us right now, which Locke concedes is less certain than the other two and declines to worry about.

The conclusion is deflationary and deliberate. Genuine knowledge covers very little. Almost everything we act on is probable opinion, and the right response is to hold opinions in proportion to the evidence for them and to be civil to people who weigh it differently. The tolerationist argument and the epistemology are the same argument, which is the point of the book.

The demonstration of God's existence in IV.x sits awkwardly in that frame, being exactly the kind of demonstrative certainty the rest of the book has been rationing, and Locke does not reconcile it.

Key passages

I.ii.1–5 — the case against innate principles.

II.i.2 — the white paper.

II.viii.9–10 — primary and secondary qualities.

II.xxiii.2 — substance as something, I know not what.

II.xxvii — personal identity through consciousness; the prince and the cobbler at §15.

III.iii.6–20 — general terms, nominal and real essence.

IV.i.2 — knowledge as the perception of agreement between ideas.

IV.xvi — degrees of assent, and the argument for proportioning belief to evidence.

IV.xviii–xix — faith, reason, and enthusiasm.

What is contested

Whether the attack on innate ideas hits anything. Leibniz's New Essays answers the Essay paragraph by paragraph and argues that Locke has refuted a doctrine nobody held. The modern version of the dispute is empirical rather than textual and has not gone Locke's way: language acquisition and infant cognition both look more structured at the start than white paper allows.

Whether the quality distinction survives. Berkeley's objection is that every argument Locke gives for the subjectivity of color works just as well against shape, and Locke has no principled place to stop. That objection has never been cleanly answered.

Whether the memory criterion works. Reid's circularity objection — you cannot define identity by memory, since remembering doing something already presupposes that you are the one who did it — is old and still live.

What the Essay is doing in the same decade as the Two Treatises. Locke's own view was that they belong together. Whether the epistemology actually supports the politics, or whether the two simply share an enemy in inherited authority, is a live question in the scholarship.