Lastly, the knowledge which I have of my will, though it is immediate, cannot be separated from that which I have of my body. I know my will, not as a whole, not as a unity, not completely, according to its nature, but I know it only in its particular acts, and therefore in time, which is the form of the phenomenal aspect of my body, as of every object. Therefore the body is a condition of the knowledge of my will. Thus, I cannot really imagine this will apart from my body. In the essay on the principle of sufficient reason, the will, or rather the subject of willing, is treated as a special class of ideas or objects. But even there we saw this object become one with the subject; that is, we saw it cease to be an object. We there called this union the miracle κατ᾽ εξοχην, and the whole of the present work is to a certain extent an explanation of this. So far as I know my will specially as object, I know it as body. But then I am again at the first class of ideas laid down in that essay, i.e., real objects. As we proceed we shall see always more clearly that these ideas of the first class obtain their explanation and solution from those of the fourth class given in the essay, which could no longer be properly opposed to the subject as object, and that, therefore, we must learn to understand the inner nature of the law of causality which is valid in the first class, and of all that happens in accordance with it from the law of motivation which governs the fourth class.
The identity of the will and the body, of which we have now given a cursory explanation, can only be proved in the manner we have adopted here. We have proved this identity for the first time, and shall do so more and more fully in the course of this work. By “proved” we mean raised from the immediate consciousness, from knowledge in the concrete to abstract knowledge of the reason, or carried over into abstract knowledge. On the other hand, from its very nature it can never be demonstrated, that is, deduced as indirect knowledge from some other more direct knowledge, just because it is itself the most direct knowledge; and if we do not apprehend it and stick to it as such, we shall expect in vain to receive it again in some indirect way as derivative knowledge. It is knowledge of quite a special kind, whose truth cannot therefore properly be brought under any of the four rubrics under which I have classified all truth in the essay on the principle of sufficient reason, § 29, the logical, the empirical, the metaphysical, and the metalogical, for it is not, like all these, the relation of an abstract idea to another idea, or to the necessary form of perceptive or of abstract ideation, but it is the relation of a judgment to the connection which an idea of perception, the body, has to that which is not an idea at all, but something toto genere different, will. I should like therefore to distinguish this from all other truth, and call it κατ᾽ εξοχην philosophical truth. We can turn the expression of this truth in different ways and say: My body and my will are one;—or, What as an idea of perception I call my body, I call my will, so far as I am conscious of it in an entirely different way which cannot be compared to any other;—or, My body is the objectivity of my will;—or, My body considered apart from the fact that it is my idea is still my will, and so forth.27
§ 19. In the first book we were reluctantly driven to explain the human body as merely idea of the subject which knows it, like all the other objects of this world of perception. But it has now become clear that what enables us consciously to distinguish our own body from all other objects which in other respects are precisely the same, is that our body appears in consciousness in quite another way toto genere different from idea, and this we denote by the word will; and that it is just this double knowledge which we have of our own body that affords us information about it, about its action and movement following on motives, and also about what it experiences by means of external impressions; in a word, about what it is, not as idea, but as more than idea; that is to say, what it is in itself. None of this information have we got directly with regard to the nature, action, and experience of other real objects.
It is just because of this special relation to one body that the knowing subject is an individual. For regarded apart from this relation, his body is for him only an idea like all other ideas. But the relation through which the knowing subject is an individual, is just on that account a relation which subsists only between him and one particular idea of all those which he has. Therefore he is conscious of this one idea, not merely as an idea, but in quite a different way as a will. If, however, he abstracts from that special relation, from that twofold and completely heterogeneous knowledge of what is one and the same, then that one, the body, is an idea like all other ideas. Therefore, in order to understand the matter, the individual who knows must either assume that what distinguishes that one idea from others is merely the fact that his knowledge stands in this double relation to it alone; that insight in two ways at the same time is open to him only in the case of this one object of perception, and that this is to be explained not by the difference of this object from all others, but only by the difference between the relation of his knowledge to this one object, and its relation to all other objects. Or else he must assume that this object is essentially different from all others; that it alone of all objects is at once both will and idea, while the rest are only ideas, i.e., only phantoms. Thus he must assume that his body is the only real individual in the world, i.e., the only phenomenon of will and the only immediate object of the subject. That other objects, considered merely as ideas, are like his body, that is, like it, fill space (which itself can only be present as idea), and also, like it, are causally active in space, is indeed demonstrably certain from the law of causality which is a priori valid for ideas, and which admits of no effect without a cause; but apart from the fact that we can only reason from an effect to a cause generally, and not to a similar cause, we are still in the sphere of mere ideas, in which alone the law of causality is valid, and beyond which it can never take us. But whether the objects known to the individual only as ideas are yet, like his own body, manifestations of a will, is, as was said in the First Book, the proper meaning of the question as to the reality of the external world. To deny this is theoretical egoism, which on that account regards all phenomena that are outside its own will as phantoms, just as in a practical reference exactly the same thing is done by practical egoism. For in it a man regards and treats himself alone as a person, and all other persons as mere phantoms. Theoretical egoism can never be demonstrably refuted, yet in philosophy it has never been used otherwise than as a sceptical sophism, i.e., a pretence. As a serious conviction, on the other hand, it could only be found in a madhouse, and as such it stands in need of a cure rather than a refutation. We do not therefore combat it any further in this regard, but treat it as merely the last stronghold of scepticism, which is always polemical. Thus our knowledge, which is always bound to individuality and is limited by this circumstance, brings with it the necessity that each of us can only be one, while, on the other hand, each of us can know all; and it is this limitation that creates the need for philosophy. We therefore who, for this very reason, are striving to extend the limits of our knowledge through philosophy, will treat this sceptical argument of theoretical egoism which meets us, as an army would treat a small frontier fortress. The fortress cannot indeed