The Fourth Book, under the head of Knowledge, treats of a great variety of interesting topics: of the nature of knowledge, its degrees, its extent, and reality; of the truth and certainty of Universal Propositions; of the logical axioms, or laws of thought; of the evidence for the existence of a God; of Faith and Reason; of the Degrees of Assent; of Enthusiasm; of Error. Into these attractive regions it is impossible that I can follow my author, but the reader who wishes to see examples of Locke's strong practical sense and, at the same time, to understand the popularity so soon and so constantly accorded to the _Essay_, should make acquaintance at least with the four chapters last named. * * * * * From the task of description I now pass to that of criticism, though this must be confined within still narrower limits than the former, and indeed, amongst the multiplicity of subjects which invite attention, I must confine myself to one only: the account of the ultimate origin of our knowledge, which forms the main subject of the _Essay_. Locke, as we have seen, derived all our knowledge from Experience. But experience, with him, was simply the experience of the individual. In order to acquire this experience, it was indeed necessary that we should have certain "inherent faculties." But of these "faculties" he gives no other account than that God has "furnished" or "endued" us with them. Thus, the _Deus ex machina_ was as much an acknowledged necessity in the philosophy of Locke, and was, in fact, almost as frequently invoked, as in that of his antagonists. Is there any natural account to be given of the way in which we come to have these "faculties," of the extraordinary facility we possess of acquiring simple and forming complex ideas, is a question which he appears never to have put to himself. Inquiries of this kind, however, we must recollect, were foreign to the men of his generation, and, in fact, have only recently become a recognized branch of mental philosophy. Hence it was that his system left so much unexplained. Not only the very circumstance that we have "inherent faculties" at all, but the wide differences of natural capacity which we observe between one man or race and another, and the very early period at which there spring up in the mind such notions as those of space, time, equality, causality, and the like, are amongst the many difficulties which Locke's theory, in its bare and unqualified form, fails satisfactorily to answer. It was thus comparatively easy for Kant to show that the problem of the origin of knowledge could not be left where Locke had left it; that our _à posteriori_ experiences presuppose and are only intelligible through certain _à priori_ perceptions and conceptions which the mind itself imposes upon them; or, to use more accurate language, through certain _à priori_ elements in our perceptions and conceptions, which the mind contributes from itself.