the sense of sight, which we chose as our example.The result of positing sensations and then constructing perception with them. Psychology has accustomed us to assume the elementary sensations corresponding to the impressions received by the rods and cones of the retina. With these sensations it goes on to reconstitute visual perception. But, in the first place, there is not one retina, there are two; so that we have to explain how two sensations, held to be distinct, combine to form a single perception corresponding to what we call a point in space.
Suppose this problem solved. The sensations in question are unextended; how will they acquire extension? Whether we see in extensity a framework ready to receive sensations, or an effect of the mere simultaneity of sensations coexisting in consciousness without coalescing, in either case something new is introduced with extensity, something unaccounted for; the process by which sensation arrives at extension, and the choice by each elementary sensation of a definite point in space, remain alike unexplained,
We will leave this difficulty, and suppose visual extension constituted. How does it in its turn reunite with tactile extension? All that my vision perceives in space is verified by my touch. Shall we say that objects are constituted by just the co-operation of sight and touch, and that the agreement of the two senses in perception may be explained by the fact that the object perceived is their common product? But how could there be
