merely empty perceptions of certain impressions made in the brain. They do not give the soul mere ideas of things, but actually place before it objects which exist outside it, although we cannot conceive how this really happens." This opinion is explained by the following facts. Although, as I have fully proved, perception is brought about by application of the law of causality, of which we are conscious a priori, yet in sight the act of the understanding, by means of which we pass from the effect to the cause, by no means appears distinctly in consciousness; and therefore the sensation does not separate itself clearly from the idea which is constructed out of it, as the raw material, by the understanding. Still less can a distinction between object and idea, which in general does not exist, appear in consciousness; but we feel the things themselves quite directly, and indeed as lying outside us, although it is certain that what is immediate can only be the sensation, and this is confined to the sphere of the body enclosed by our skin. This can be explained from the fact that outside us is exclusively a spatial determination. But space itself is a form of our faculty of perception, i.e., a function of our brain. Therefore that externality to us to which we refer objects, on the occasion of sensations of sight, is itself really within our heads; for that is its whole sphere of activity. Much as in the theatre we see the mountains, the woods, and the sea, but yet everything is inside the house. From this it becomes intelligible that we perceive things in the relation of externality, and yet in every respect immediately, but have not within us an idea of the things which lie outside us, different from these things. For things are in space, and consequently also external to us only in so far as we perceive them. Therefore those things which to this extent we perceive directly, and not mere images of them, are themselves only our ideas, and as such exist only in our heads. Therefore we do not, as Euler says, directly perceive the things themselves which are external to us, but rather the things
