Locke opened a new way for English philosophy. Stillingfleet saw dangers ahead in that way; but its discovery was Locke’s title to fame. It was no new thing, certainly, to lay stress upon method. Herein, he followed the example of Bacon and Hobbes and other pioneers of modern philosophy. Bacon had done more: he had found dangers and defects in the natural working of men’s minds, and had devised means to correct them. But Locke went a step further, and undertook a systematic investigation of the human understanding with a view to determining something else—namely, the truth and certainty of knowledge, and the grounds of belief, on all matters about which men are in the habit of making assertions. In this way he introduced a new department, or a new method, of philosophical enquiry, which has come to be known as theory of knowledge, or epistemology; and, in this respect, he was the precursor of Kant and anticipated what Kant called the critical method.
We have Locke’s own account of the origin of the problem in his mind. He struck out a new way because he found the old paths blocked. Five or six friends were conversing in his room, probably in London and in the winter of 1670–1, “on a subject very remote from this”; the subject, as we learn from another member of the party, was the “principles of morality and revealed religion”; but difficulties arose on every side, and no progress was made. Then, he goes on to say,
it came into my thoughts that we took a wrong course, and that before we set ourselves upon inquires of that nature, it was necessary to examine our own abilities, and see what objects our understandings were, or were not, fitted to deal with.
At the request of his friends, Locke agreed to set down his thoughts on this question against their next meeting; and he expected that a single sheet of paper would suffice for the purpose. So little did he realise the magnitude of the issues which he raised and which were to occupy his leisure for nearly twenty years.
Locke’s interest centres in the traditional problems—the nature of self, the world and God, and the grounds of our knowledge of them. We reach these questions only in the fourth and last book of the Essay. But to them the enquiry of the first three books is preliminary, though it has, and Locke saw that it had, an importance of its own. His introductory sentences make this plain:
Since it is the understanding that sets man above the rest of sensible beings, and gives him all the advantage and dominion which he has over them; it is certainly a subject, even for its nobleness, worth our labour to inquire into. The understanding, like the eye, while it makes us see and perceive all other things, takes no notice of itself; and it requires art and pains to set it at a distance and make it its own object. But whatever be the difficulties that lie in the way of this inquiry; whatever it be that keeps us so much in the dark to ourselves; sure I am that all the light we can let in upon our minds, all the acquaintance we can make with our own understandings, will not only be very pleasant, but bring us great advantage, in directing our thoughts in the search of other things.
Locke will not “meddle with the physical consideration of the mind”; he has no theory about its essence or its relation to the body; at the same time, he has no doubt that, if due pains be taken, the understanding can be studied like anything else: we can observe its objects and the ways in which it operates upon them. All the objects of the understanding are described as ideas, and ideas are spoken of as being in the mind. 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 term in modern psychology 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, percept, 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, lay a danger from which he did not escape. In common usage “idea” carries with it a suggestion of contrast with reality; and the opposition which the “new way of ideas” excited was due to the doubt which it seemed to cast on the claim of knowledge to be a knowledge of real things.