No doubt the theory lying at the root of Egoism is, from the =empirical standpoint=, perfectly justified. From the testimony of experience, the =distinction= between one's own person and that of another appears to be absolute. I do not occupy the same space as my neighbour, and this difference, which separates me from him physically, separates me also from his weal and woe. But in the first place, it should be observed that the knowledge we have Of our own selves is by no means exhaustive and transparent to its depths. By means of the intuition, which the brain constructs out of the data supplied by the senses, that is to say, in an indirect manner, we recognise our body as an object in space; through an inward perception, we are aware of the continuous series of our desires, of our volitions, which arise through the agency of external motives; and finally, we come to discern the manifold movements, now stronger, now weaker, of our will itself, to which all feelings from within are ultimately traceable. And that is all: =for the perceiving faculty is not in its turn perceived=. On the contrary, the real substratum of our whole phaenomenal nature, our inmost essence in itself, that which wills and perceives, is not accessible to us. We see only the outward side of the ego; its inward part is veiled in darkness. Consequently, the knowledge we possess of ourselves is in no sort radical and complete, but rather very superficial. The larger and more important part of our being remains unknown, and forms a riddle to speculate about; or, as Kant puts it: "The ego knows itself only as a phaenomenon; of its real essence, whatever that may be, it has no knowledge." Now, as regards that side of the self which falls within our ken, we are, undoubtedly, sharply distinguished, each from the other; but it does not follow therefrom that the same is true of the remainder, which, shrouded in impenetrable obscurity, is yet, in fact, the very substance of which we consist. There remains at least the possibility that the latter is in all men uniform and identical. What is the explanation of all plurality, of all numerical diversity of existence? Time and Space. Indeed it is only through the latter that the former is possible: because the concept "many" inevitably connotes the idea either of succession (time), or of relative position (space). Now, since a homogeneous plurality is composed of =Individuals=, I call Space and Time, as being the conditions of multiplicity, the _principium individuationis_ (the principle of individuation); and I do not here pause to consider whether this expression was exactly so employed by the Schoolmen.