This truth is by no means new. It was implicitly involved in the sceptical reflections from which Descartes started. Berkeley, however, was the first who distinctly enunciated it, and by this he has rendered a permanent service to philosophy, even though the rest of his teaching should not endure. Kant’s primary mistake was the neglect of this principle, as is shown in the appendix. How early again this truth was recognised by the wise men of India, appearing indeed as the fundamental tenet of the Vedânta philosophy ascribed to Vyasa, is pointed out by Sir William Jones in the last of his essays: “On the philosophy of the Asiatics” (Asiatic Researches, vol. iv. p. 164), where he says, “The fundamental tenet of the Vedanta school consisted not in denying the existence of matter, that is, of solidity, impenetrability, and extended figure (to deny which would be lunacy), but in correcting the popular notion of it, and in contending that it has no essence independent of mental perception; that existence and perceptibility are convertible terms.” These words adequately express the compatibility of empirical reality and transcendental ideality. 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 any one accepts the world as merely his idea, warns him that this view of it, however true it may be, is nevertheless one-sided, adopted 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; a truth at which we can arrive only by deeper research and more severe abstraction, by the separation of what is different and the union of what is identical. This truth, which must be very serious and impressive if not awful to every one, is that a man can also say and must say, “the world is my will.” In this book, however, we must consider separately that aspect of the world from which we start, its aspect as knowable, and therefore, in the meantime, we must, without reserve, regard all presented objects, even our own bodies (as we shall presently show more fully), merely as ideas, and call them merely ideas. By so doing we always abstract from will (as we hope to make clear to every one further on), 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.