All the objects of the understanding are described as ideas, and ideas are spoken of as being in the mind (Intro. 2; Bk. 2:1:5; Bk. 2:8:8). Locke's first problem, therefore, is to trace the origin and history of ideas, and the ways in which the understanding operates upon them, in order that he may be able to see what knowledge is and how far it reaches. This wide use of the term "idea" is inherited from Descartes. The contemporary term which corresponds with it most nearly is "presentation". But presentation is, strictly, only one variety of Locke's idea, which includes also representation and image, perception, and concept or notion. His usage of the term thus differs so widely from the old Platonic meaning that the danger of confusion between them is not great. It suited the author's purpose also from being a familiar word in ordinary discourse as well as in the language of philosophers. Herein, however, lays danger from which he did not escape. In common usage "idea" carries with it a suggestion of contrast with reality; this is not supposed in Locke's use.
In the first book of the Essay, on the subject of innate ideas, Locke points to the variety of human experience, and to the difficulty of forming general and abstract ideas, and he ridicules the view that any such ideas can be antecedent to experience. All the parts of our knowledge, he insists, have the same rank and the same history regarding their origin in experience. It is in its most extreme form that the doctrine of innate ideas is attacked; but he cannot seen any middle ground between that extreme doctrine and his own view that all ideas have their origin in experience. Indeed, it is difficult to determine against whom the argument is directed. But when we note Locke's polemical interest in the question, and remember the significance for him of the empirical origin of all the elements of human knowledge, we can be content to see in it an earnest protest against the principle of authority, a vindication of our right to examine critically all the so-called "principles" of human knowledge.
Locke wishes to avoid any presupposition about matter, or mind, or their relation. It is not difficult to see that the notions which he has expelled often re-enter. But the peculiar value of his approach consists in his attempt to keep clear of them. He begins neither with ind nor matter, but with ideas. Their existence needs no proof: "everyone is conscious of them in himself, and men's words and actions will satisfy him that they are in others." His first inquiry is "how they come into the mind"; has next business is to show that they constitute the whole material of our knowledge. In his answer to the former question we discover the influence of traditional philosophy, or rather of ordinary common sense views of existence, upon his views. All our ideas, he says, come from experience. The mind has no innate ideas, but it has innate faculties: it perceives, remembers, and combines the ideas that come to it from without; it also desires, deliberates, and wills; and these mental activities are themselves the source of a new class of ideas. Experience is therefore twofold. Our observation may be employed either about external sensible objects, or about the internal operations of our minds. The former is the source of most of the ideas which we have, and, as it depends "wholly upon our senses," is called "sensation." The latter is a source of ideas which "every man has wholly in himself," and it might be called "internal sense"; to it he gives the name "reflection."