These “simple” ideas come into the crucible of the mind and form “complex” ideas of various sorts. There are three general classes of these complex ideas: substances, modes, and relations. The construction of “complex ideas” out of “simple ideas” and the objects to which the complex ideas refer receive a great variety of illustration at Locke’s hands, but the details of his lengthy discussion need not detain us. He is very painstaking; he shows hard common sense; but he is deficient in logical classification and he often betrays much indecision. His _Essay_ is of encyclopædic character in its derivation of all common notions from “simple ideas.” The laws of association form the chemistry by which he welds the “simple ideas” together. Thus far Locke is empirical and consistent. However, the dualistic background of the thought of his age makes him deviate from his avowed empiricism. Besides the clear and simple ideas of sensation and reflection Locke introduces the idea of the Self. What is the idea of Self? It is not a sensation nor a reflection. It is not a complex idea, derived from sensations and reflections. “It is an internal, infallible perception that we are.” It is an accompaniment of the processes of thought. It stands beside the ideas, which are empirically derived, as an unexplained remainder. The result of Locke’s psychological analysis is therefore that the inner world of the mind consists of the combination of the simple ideas of sensation and reflection plus the unexplained idea of the Self. =Locke’s Theory of Knowledge.= Although Locke says that the purpose of his _Essay_ is to show the limits and extent of human knowledge, he does not reach this until the last book. The first three books form a long introduction to the fourth book and his real theme. Here for the first time he treats the mind as active; and here for the first time in the history of thought the attempt is made to show what questions man can answer with certainty, what with probability, and what are beyond man’s knowledge.