"The world is my idea:"--this is 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; that the world which surrounds him is there only as 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; and each of these, which we have seen to be just so many modes of the principle of sufficient reason, is valid only for a particular class of ideas; whereas the antithesis of object and subject is the common form of all these classes, is that form under which alone any idea of whatever kind it may be, abstract or intuitive, pure or empirical, is possible and thinkable. No truth therefore is more certain, more independent of all others, and less in need of proof than this, that all that exists for knowledge, and therefore this whole world is only obect in relation to subject, perception of a perceiver, in a word, idea. This is obviously true of the past and the future, as well as of the present, of what is said farthest off, as of what is near; for it is true of time and space themselves, in which alone these distinctions arise. All that in any way belongs or can belong to the world is inevitably thus conditioned through the subject, and exists only for the subject. The world is idea.
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In this first book, then, we consider the world only from this side, only so far as it is idea. The inward reluctance with which anyone accepts the world as merely his idea, warns him that this view of it, however true it may be, is nevertheless one-sided, adoped in consequence of some arbitrary abstraction. And yet it is a conception from which he can never free himself. The defectiveness of this view will be corrected in the next book by means of a truth which is not so immediately certain as that from which we start here . . . This truth, which must be very serious and impressive if not awful to everyone, is that a man can also say and must say, "the world is my will."
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[W]e always abstract from will . . . which by itself constitutes the other aspect of the world. For as the world is in one aspect entirely idea, so in another it is entirely will. A reality which is neither of these two, but an object in itself (into which the thing in itself has unfortunately dwindled in the hands of Kant), is the phantom of a dream, and its acceptance is an ignis fatuus in philosophy.
--Arthur Schopenhauer, "The World as Will and Idea: First Book" (The Age of Ideology, Henry D. Aiken, pp. 104-106)
In fact, the meaning for which we seek of that world which is present to us only as our idea, or the transition from the world as mere idea of the knowing subject to whatever it maybe besides this, would never be found if the investigator himself were nothing more than a pure knowing subect (a winged cherub without a body). But he is himself rooted in that world; he finds himself in it as an individual, that is to say, his knowledge, which is the necessary supporter of the whole world as idea, is yet always given through the medium of a body, whose affections are . . . the starting point for understanding in the perception of that world.
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[W]ill. This and this alone gives him [the individual] the key to his own existence, reveals to him the significance, shows him the inner mechanism of his being, of his action, of his movements. The body is given in two entirely different ways to the subject of knowledge, who becomes an individual only through his identitity with it. It is given as an idea in intelligent perception, as an object among objects and subject to the laws of objects. And it is also given in quite a different way as that which is immediately known to everyone, and is signified by the word will. Every true act of his will is also at once and without exception a movement of his body. The act of will and the movement of the body are not two different things objectively known, which the bond of causality unites; they do not stand in the relation of cause and effect; they are one and the same, but they are given in entirely different ways--immediately, and again in perception for the understanding. The action of the body is nothing but the act of the will objectified, i.e., passed into perception. It will appear later that this is true of every movement of the body, not merely those which follow upon motives, but also involuntary movements which follow upon mere stimuli, and indeed, that the whole body is nothing but objectified will, i.e., will become idea.
It is only in reflection that to will and to act are different; in reality they are one.
--Arthur Schopenhauer, "The World as Will and Idea: Second Book" (The Age of Ideology, Henry D. Aiken, pp. 107-109)