Still less is it legitimate to represent the principle that "the perceptual world is my idea" as self-evident and needing no proof. Schopenhauer begins his chief work, The World as Will and Idea, with the words: "The world is my idea--this is a truth which holds good for everything that lives and knows, though man alone can bring it into reflective and abstract consciousness. If he really does this, he has attained to philosophical wisdom. It then becomes clear and certain to him that what he knows is not a sun and an earth, but only an eye that sees a sun, a hand that feels an earth; that the world which surrounds him is there only in idea, i.e., only in relation to something else, the consciousness which is himself. If any truth can be asserted a priori, it is this: for it is the expression of the most general form of all possible and thinkable experience, a form which is more general than time, or space, or causality, for they all presuppose it ..." (The World as Will and Idea, Book I, par. 1). This whole theory is wrecked by the fact, already mentioned above, that the eyes and the hand are just as much percepts as the sun and the earth. Using Schopenhauer's vocabulary in his own sense, I might maintain against him that my eye which sees the sun, and my hand which feels the earth, are my ideas just like the sun and the earth themselves. That, put in this way, the whole theory cancels itself, is clear without further argument. For only my real eye and my real hand, but not my ideas "eye" and "hand," could own the ideas "sun" and "earth" as modifications. Yet it is only in terms of these ideas that Critical Idealism has the right to speak. Critical Idealism is totally unable to gain an insight unto the relation of percept to idea. It cannot make the separation, mentioned on p. 58, between what happens to the percept in the process of perception and what must be inherent in it prior to perception. We must therefore attempt this problem in another way. V OUR KNOWLEDGE OF THE WORLD