The most difficult to drive from the field are the so-called physiological proofs of the subjectivity of our percepts. When I exert pressure on the skin of my body, I experience it as a pressure sensation. This same pressure can be sensed as light by the eye, as sound by the ear. I experience an electrical shock by the eye as light, by the ear as sound, by the nerves of the skin as touch, and by the nose as a smell of phosphorus. What follows from these facts? Only this: I experience an electrical shock, or, as the case may be, a pressure followed by a light, or a sound, or, it may be, a certain smell, etc. If there were no eye present, then no light quality would accompany the perception of the mechanical vibrations in my environment; without the presence of the ear, no sound, etc. But what right have we to say that in the absence of sense-organs the whole process would not exist at all? All those who, from the fact that an electrical process causes a sensation of light in the eye, conclude that what we sense as light is only a mechanical process of motion, forget that they are only arguing from one percept to another, and not at all to something altogether transcending percepts. Just as we can say that the eye perceives a mechanical process of motion in its surroundings as light, so we can affirm that every change in an object, determined by natural law, is perceived by us as a process of motion. If I draw twelve pictures of a horse on the circumference of a rotating disc, reproducing exactly the positions which the horse's body successively assumes in movement, I can, by rotating the disc, produce the illusion of movement. I need only look through an opening in such a way that, at regular intervals, I perceive the successive positions of the horse. I perceive, not separate pictures of twelve horses, but one picture of a single galloping horse. The above-mentioned physiological facts cannot, therefore, throw any light on the relation of percept to idea. Hence, we must seek a relation some other way.