We have seen that scientific explanation does not go beyond presentations ordered in space and time. This is just as true of the sciences of causation--the 'ætiological' sciences--as it is of mathematical science. All that we learn from Mechanics, Physics, Chemistry and Physiology, is 'how, in accordance with an infallible rule, one determinate state of matter necessarily follows another: how a determinate change necessarily conditions and brings on another determinate change.' This knowledge does not satisfy us. We wish to learn the significance of phenomena; but we find that from outside, while we view them as presentations, their inner meaning is for ever inaccessible.
The starting-point for the metaphysical knowledge we seek is given us in our own body. The animal body is 'the immediate object of the subject': in it as presentation the 'effects' of 'causes' in the order of presentations external to it are first recognised. Now in virtue of his body the investigator is not pure knowing subject standing apart from that which he knows. In the case of the particular system of presentations constituting his organism, he knows what these presentations signify, and that is his _will_ in a certain modification. The subject appears as individual through its identity with the body, and this body is given to it in two different ways: on one side as object among objects, and subjected to their laws; on the other side as the will immediately known to each. The act of will and the movement of the body are not two different states related as cause and effect; for the relation of cause and effect belongs only to the object, the phenomenon, the presentation. They are one and the same act given in different manners: the will, immediately to the subject; the movement, in sensible intuition for understanding. The action of the body is the objectified act of will. Called at first the immediate object of presentation, the body may now, from the other side, be called 'the objectivity of the will.'
Thus, as was said, the 'law of motivation' discloses the inner nature of causality. In causality in general we know only relations of phenomena; but in the case of our own body we know something else that those relations express; namely, the act of will determined by motives. Now there are in the world as presentation other systems like that which we call our body. Unless all these are to be supposed mere phantoms without inner reality, we must infer by analogy, in correspondence with like phenomena, other individual wills similar to that which we know in ourselves. This inference from analogy, universally admitted in the case of human and animal bodies, must be extended to the whole corporeal world. The failure to take this step is where the purely intellectual forms of idealism have come short. Kant's 'thing-in-itself,' which is not subject to the forms by which presentations become experience, but which experience and its forms indicate as the reality, has been wrongly condemned by his successors as alien to idealism. It is true that Kant did in some respects fail to maintain the idealistic position with the clearness of Berkeley; but his shortcoming was not in affirming a thing-in-itself beyond phenomena. Here, in Schopenhauer's view, is the metaphysical problem that he left a place for but did not solve. The word of the riddle has now been pronounced. Beyond presentation, that is, in itself and according to its innermost essence, the world is that which we find in ourselves immediately as will. By this it is not meant that a falling stone, for example, acts from a motive; knowledge and the consequent action from motives belongs only to the determinate form that the will has in animals and men; but the reality in the stone also is the same in essence as that to which we apply the name of will in ourselves. He who possesses this key to the knowledge of nature's innermost being will interpret the forces of vegetation, of crystallisation, of magnetism, of chemical affinity, even of weight itself, as different only in phenomenal manifestation but in essence the same; namely, that which is better known to each than all else, and where it emerges most clearly is called will. Only the will is thing-in-itself. It is wholly different from presentation, and is that of which presentation is the phenomenon, the visibility, the objectivity. Differences affect only the degree of the appearing, not the essence of that which appears.