Locke's Essay is part of the so-called epistemological turn given philosophy by Descartes that assigned fundamental importance to the theory of knowledge. So much is clear from the introduction to Book I: “my Purpose [is] to enquire into the Original, Certainty, and Extent of humane Knowledge; together, with the Grounds and Degrees of Belief, Opinion, and Assent” (E I.i.2: 43). The general aim in the period was to reconcile the startling discrepancy, in all its ramifications, between the so-called manifest image, as it has come to be called, which views the world in the commonsense terms of colors, odors, and tastes, and the emerging scientific image, which sees it in the atomistic (or “corpuscularian”) terms of sizes, shapes, and motion.

Locke's version of the epistemological turn took what his ecclesiastical critic Stillingfleet called the new way of ideas: we perceive things not as they are in themselves, but in terms of our ideas of them. By an idea Locke means the mind's immediate object whenever it thinks, which for him is something in the mind (E I.i.8; II.viii.8). The mind knows the mediate object outside the mind insofar as the idea represents it. The distinction between an idea and its object is Locke's way of dealing with the discrepancy between the manifest and scientific images. So far, all interpretations of Locke are in basic agreement.