The Metaphysics of Morals by Immanuel Kant Chapter 3 Page 16

that he can obtain his knowledge even of himself only by the inner sense and, consequently, only through the appearances of his nature and the way in which his consciousness is affected. At the same time beyond these characteristics of his own subject, made up of mere appearances, he must necessarily suppose something else as their basis, namely, his ego, whatever its characteristics in itself may be.

Thus in respect to mere perception and receptivity of sensations he must reckon himself as belonging to the world of sense; but in respect of whatever there may be of pure activity in him (that which reaches consciousness immediately and not through affecting the senses), he must reckon himself as belonging to the intellectual world, of which, however, he has no further knowledge. To such a conclusion the reflecting