An Essay Concerning Human Understanding is a work by John Locke concerning the foundation of human knowledge and understanding. It first appeared in 1689 (although dated 1690) with the printed title An Essay Concerning Humane Understanding. He describes the mind at birth as a blank slate (tabula rasa, although he did not use those actual words) filled later through experience. The essay was one of the principal sources of empiricism in modern philosophy, and influenced many enlightenment philosophers, such as David Hume and George Berkeley.

Book I of the Essay is Locke's attempt to refute the rationalist notion of innate ideas. Book II sets out Locke's theory of ideas, including his distinction between passively acquired simple ideas—such as "red", "sweet", "round"—and actively built complex ideas, such as numbers, causes and effects, abstract ideas, ideas of substances, identity, and diversity. Locke also distinguishes between the truly existing primary qualities of bodies, like shape, motion and the arrangement of minute particles, and the secondary qualities that are "powers to produce various sensations in us" such as "red" and "sweet." These secondary qualities, Locke claims, are dependent on the primary qualities. He also offers a theory of personal identity, offering a largely psychological criterion. Book III is concerned with language, and Book IV with knowledge, including intuition, mathematics, moral philosophy, natural philosophy ("science"), faith, and opinion.

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." 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, in his words, an "established opinion" 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.

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. 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."

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." 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." 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.

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