All the difficulties in the assumptions of the Enlightenment come out in Locke’s treatment of his main theme. Locke defines knowledge as the “perception of the agreement or disagreement of our ideas,” and yet he says that knowledge is real only as ideas agree with things. That is to say, Locke had assumed (in Book II of the _Essay_) the existence of the material substance of things of the outer world, just as he assumed the existence of the spiritual Self-substance of the inner world. What is the nature of the outer material substance? Locke hesitates, and the best he can answer is, “It is the unknown support and cause of the union of several distinct, simple ideas.” Substance, to Locke, is a word for something unknown. But does the mind know nothing about substance? What information do our ideas convey to us of substance? We have this knowledge: we know the _primary_ or constant, unchangeable qualities of substances, and the _secondary_ or variable qualities of substances. The _primary_ qualities of bodies are the same as their effects in us, such as the extension of bodies, their solidity, movement and rest, duration and position in time. The _secondary_ “are nothing in the objects themselves but powers to produce various sensations in us by their primary qualities.” Secondary qualities are sounds, colors, etc. In this confused statement it would seem that substance stands as merely the nominal support of the _primary_ qualities, and the _primary_ qualities are the cause of the _secondary_ qualities. Thus the individual stands forth free in the development of his ideas, but he is an individual circumscribed by his dualistic world. He belongs to the world of an unexplained spiritual substance on the one hand, and he is surrounded by a world of an unknown material substance on the other. There are three kinds of knowledge: intuitive, demonstrative, and probable. Locke says that the individual is intuitively certain of his own ideas. The individual has also demonstrative knowledge――he can reason logically and mathematically. But Locke’s real problem does not lie with intuitive and demonstrative knowledge. The question that concerned him was rather, What is the character of our knowledge of the external world? The individual in the Enlightenment lived in a spiritual independence of matter, yet he had a feeling of uncertainty about his hold upon a world of matter so different from himself. It was a world foreign to his spiritual essence. With the deepening of the mind within itself and with its growing independence, the equally independent material world grew more difficult and distant. Locke feels this difficulty. How can man know this external world? How can the individual, with all his freedom, bring the external world under his control?