Book I

The main thesis is that there are "No Innate Principles." Locke wrote, "If we will attentively consider new-born children, we shall have little reason to think, that they bring many ideas into the world with them." Rather, "by degrees, afterwards, ideas come into their minds; and...they get no more, nor no other, than what experience, and the observation of things, that come in their way, furnish them with."^[2] Book I of the Essay is an attack on nativism or the doctrine of innate ideas; Locke indeed sought to rebut a prevalent view of innate ideas that was firmly held by philosophers of his time. While he allowed that some ideas are in the mind from an early age, he argued that those ideas are furnished by the senses starting in the womb—for instance, differences between colours or tastes. If we have a universal understanding of a concept like sweetness, it is not because this is an innate idea, but because we are all exposed to sweet tastes at an early age.^[3]

One of Locke's fundamental arguments against innate ideas is the very fact that there is no truth to which all people attest. He took the time to argue against a number of propositions that rationalists offer as universally accepted truth, for instance the principle of identity, pointing out that at the very least children and idiots are often unaware of these propositions.^[4] In anticipating a counterargument, namely the use of reason to comprehend already existent innate ideas, Locke states that "by this means, there will be no difference between the maxims of the mathematicians, and theorems they deduce from them; all must be equally allowed innate; they being all discoveries made by the use of reason."^[5]

Book II

Whereas Book I is intended to reject the doctrine of innate ideas proposed by Descartes and the rationalists, Book II explains that every idea is derived from experience either by sensation—i.e. direct sensory information—or reflection—i.e. "the perception of the operations of our own mind within us, as it is employed about the ideas it has got."

In Book II, Locke focuses on the ideas of substances and qualities, in which the former are "an unknown support of qualities" and latter have the "power to produce ideas in our mind."^[6] Substance is what holds qualities together, while qualities themselves allow us to perceive and identify objects. A substance consists of bare particulars and does not have properties in themselves except the ability to support qualities. Substances are "nothing but the assumption of an unknown support for a group of qualities that produce simple ideas in us."^[7] Despite his explanation, the existence of substances is still questionable as they cannot necessarily be "perceived" by themselves and can only be sensed through the qualities.

In terms of qualities, Locke divides such into primary and secondary, whereby the former give our minds ideas based on sensation and actual experience. In contrast, secondary qualities allow our minds to understand something based on reflection, in which we associate what we perceive with other ideas of our own.^[8]

Furthermore, Book II is also a systematic argument for the existence of an intelligent being:

Thus, from the consideration of ourselves, and what we infallibly find in our own constitutions, our reason leads us to the knowledge of this certain and evident truth, that there is an eternal, most powerful, and most knowing being; which whether any one will please to call God, it matters not!

Locke contends that consciousness is what distinguishes selves, and thus,^[9]

…in this alone consists personal Identity, i.e. the sameness of rational Being: And 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 it was then; and 'tis by the same self with this present one that now reflects on it, that that Action was done.

Book III

Book III focuses on words. Locke connects words to the ideas they signify, claiming that man is unique in being able to frame sounds into distinct words and to signify ideas by those words, and then that these words are built into language.

Chapter ten in this book focuses on "Abuse of Words." Here, Locke criticizes metaphysicians for making up new words that have no clear meaning. He also criticizes the use of words which are not linked to clear ideas, and to those who change the criteria or meaning underlying a term.

Thus, Locke uses a discussion of language to demonstrate sloppy thinking, following the Port-Royal Logique (1662)^[10] in numbering among the abuses of language those that he calls "affected obscurity" in chapter 10. Locke complains that such obscurity is caused by, for example, philosophers who, to confuse their readers, invoke old terms and give them unexpected meanings or who construct new terms without clearly defining their intent. Writers may also invent such obfuscation to make themselves appear more educated or their ideas more complicated and nuanced or erudite than they actually are.

Book IV

This book focuses on knowledge in general—that it can be thought of as the sum of ideas and perceptions. Locke discusses the limit of human knowledge, and whether such can be said to be accurate or truthful.

Thus, there is a distinction between what an individual might claim to know, as part of a system of knowledge, and whether or not that claimed knowledge is actual. Locke writes at the beginning of the fourth chapter ("Of the Reality of Knowledge"):

I doubt not but my Reader by this Time may be apt to think that I have been all this while only building a Castle in the Air; and be ready to say to me, To what purpose all this stir? Knowledge, say you, is only the Perception of the Agreement or Disagreement of our own Ideas: but who knows what those Ideas may be?… But of what use is all this fine Knowledge of Men's own Imaginations, to a Man that enquires after the reality of things? It matters not what Men's Fancies are, 'tis the Knowledge of Things that is only to be priz'd; 'tis this alone gives a Value to our Reasonings, and Preference to one Man's Knowledge over another's, that it is of Things as they really are, and not of Dreams and Fancies.

In the last chapter of the book, Locke introduces the major classification of sciences into natural philosophy, semiotics, and ethics.