Objective perception makes use, properly speaking, of only two senses ; touch and sight. These alone supply the data upon which, as its basis, the Understanding constructs the objective world by the process just described. The three other senses remain on the whole subjective; for their sensations, while pointing to an external cause, still contain no data by which its relations in Space can be determined. Now Space is the form of all perception, i.e. of that apprehension, in which alone objects can, properly speaking, present themselves. Therefore those other three senses can no doubt serve to announce the presence of objects we already know in some other way ; but no construction in Space, consequently no objective perception, can possibly be founded on their data. A rose cannot be constructed from its perfume, and a blind man may hear music all his life without having the slightest objective representation either of the musicians, or of the instruments, or of the vibrations of the air. On the other hand, the sense of hearing is of great value as a medium for language, and through this it is the sense of Reason. It is also valuable as a medium for music, which is the only way in which we comprehend numerical relations not only in abstracto, but directly, in concreto. A musical sound or tone, however, gives no clue to spacial relations, therefore it never helps to bring the nature of its cause nearer to us ; we stop short at it, so that it is no datum for the Understanding in its construction of the objective world. The sensations of touch and sight alone are such data ; therefore a blind man without either hands or feet, while able to construct Space for himself a priori in all its regularity, would nevertheless acquire but a very vague representation of the objective world. Yet what is supplied by touch and sight is not by any means perception, but merely the raw material for it. For perception is so far from being contained in the sensations of touch and sight, that these sensations
