176 THE WORLD AS WILL. BK. II.
world as idea) exists only for the understanding; the understanding is its condition, its supporter as its necessary correlative. I repeat all this in passing, merely to call to mind what was demonstrated in the First Book, for it is necessary for the complete understanding of these two books that their inner agreement should be observed, since what is inseparably united in the actual world as its two sides, will and idea, has, in order that we might understand each of them more clearly in isolation, been dissevered in these two books.
It may not perhaps be superfluous to elucidate further by an example how the law of causality has meaning only in relation to time and space, and the matter which consists in the union of the two. For it determines the limits in accordance with which the phenomena of the forces of nature divide themselves in the possession of matter, while the original forces of nature, as the immediate objectification of will, which, as a thing in itself, is not subordinated to the principle of sufficient reason, lie outside these forms, within which alone all etiological explanation has validity and meaning, and just on that account can never lead us to the inner reality of nature. For this purpose let us think of some kind of machine constructed according to the laws of mechanics. Iron weights begin the motion by their gravity; copper wheels resist by their rigidity, affect and raise each other and the lever by their impenetrability, and so on. Here gravity, rigidity, and impenetrability are original unexplained forces; mechanics only gives us the condition under which, and the manner in which, they manifest themselves, appear, and govern a definite matter, time, and place. If, now, a strong magnet is made to attract the iron of the weight, and overcome its gravity, the movement of the machine stops, and the matter becomes forthwith the scene of quite a different force of nature — magnetism, of which etiology again gives no further explanation than the condition under which it appears.
