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 my skin I perceive it as a pressure sensation. This same pressure can be sensed as light by the eye, as sound by the ear. An electric shock is perceived by the eye as light, by the ear as noise, by the nerves of the skin as impact, and by the nose as a phosphoric smell. What follows from these facts? Only this: I perceive an electric shock (or a pressure, as the case may be) followed by an impression of light, or sound, or perhaps a certain smell, and so on. If there were no eye present, then no perception of light would accompany the perception of the mechanical disturbance in my environment; without the presence of the ear, no perception of sound, and so on. But what right have we to say that in the absence of sense organs the whole process would not exist at all? Those who, from the fact that an electrical process calls forth light in the eye, conclude that what we sense as light is only a mechanical process of motion when outside our organism, forget that they are only passing from one percept to another, and not at all to something lying beyond percepts. Just as we can say that the eye perceives a mechanical process of motion in its surroundings as light, so we could equally well say that a regular and systematic change in an object 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 attitudes which the horse's body successively assumes when galloping, I can produce the illusion of movement by rotating the disc. I need only look through an opening in such a way that, in the proper intervals, I see the successive positions of the horse. I do not see twelve separate pictures of a horse but the picture of a single galloping horse.
 The physiological fact mentioned above cannot therefore throw any light on the relation of percept to mental picture. We must go about it rather differently.