with divers splashes of colour. Nor would anything more remain over in our consciousness, were we suddenly deprived of all our Understanding—let us say by paralysis of the brain—at a moment when we were contemplating a rich and extensive landscape, while the sensation was left unchanged : for this was the raw material out of which our Understanding had just before been constructing that perception.
Now, that the Understanding should thus be able, from such limited material as light, shade and colour, to produce the visible world, inexhaustibly rich in all its different shapes, by means of the simple function of referring effects to causes assisted by the intuition of Space, depends before all things upon the assistance given by the sensation itself, which consists in this : first, that the retina, as a surface, admits of a juxtaposition of impressions ; secondly, that light always acts in straight lines, and that its refraction in the eye itself is rectilinear ; finally, that the retina possesses the faculty of immediately feeling from which direction the light comes that impinges upon it, and this can, perhaps, only be accounted for by the rays of light penetrating below the surface of the retina. But by this we gain, that the mere impression at once indicates the direction of its cause ; that is, it points directly to the position of the object from which the light proceeds or is reflected. The passage to this object as a cause no doubt presupposes the knowledge of causal relations, , as well as of the laws of Space ; but this knowledge constitutes precisely the furniture of the Intellect, which, here also, has again to create perception out of mere sensation. Let us now examine its procedure in doing so more closely.
The first thing it does is to set right the impression of the object, which is produced on the retina upside down. That original inversion is, as we know, brought about in the following manner. As each point of the visible object sends
