The feeling of the insufficiency of a purely sensationalistic explanation of perception is in like manner shown in the assertion which was made shortly before the appearance of the Kantian philosophy, that we not only have _ideas_ of things called forth by sensation, but apprehend the _things themselves_ directly, although they lie outside us—which is certainly inconceivable. And this was not meant in some idealistic sense, but was said from the point of view of common realism. This assertion is well and pointedly put by the celebrated Euler in his “Letters to a German Princess,” vol. ii. p. 68. He says: “I therefore believe that the sensations (of the senses) contain something more than philosophers imagine. They are not 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 which are perceived by us as external to us are only our ideas, and consequently are apprehended by us immediately.