the eye does not, at the outset, give us any information as to any of the relations of space. Visual form, visual relief, visual distance, then become the symbols of tactile perceptions. But how is it, then, that this symbolism succeeds? Here are objects which change their shape and move. Vision takes note of definite changes which touch afterwards verifies. There is, then, in the two series, visual and tactile, or in their causes, something which makes them correspond one to another and ensures the constancy of their parallelism. What is the principle of this connexion?
For English idealism, it can only be some deus ex machina, and we are confronted with a mystery again. For ordinary realism, it is in a space distinct from the sensations themselves that the principle of the correspondence of sensations one with another lies; but this doctrine only throws the difficulty further back and even aggravates it, for we shall now want to know how a system of homogeneous movements in space evokes various sensations which have no resemblance whatever with them. Just now the genesis of visual perception of space by a mere association of images appeared to us to imply a real creation ex nihilo; here all the sensations are born of nothing, or at least have no resemblance with the movement that occasions them. In the main, this second theory differs much less from the first than is commonly believed.
