It is extraordinarily difficult to divest ourselves of the belief that the physical world is the world we perceive by sight and touch; even if, in our philosophic moments, we are aware that this is an error, we nevertheless fall into it again as soon as we are off our guard. The notion that what we see is “out there” in physical space is one which cannot survive while we are grasping the difference between what physics supposes to be really happening, and what our senses show us as happening; but it is sure to return and plague us when we begin to forget the argument. Only long reflection can make a radically new point of view familiar and easy. Our illustrations hitherto have been taken from the sense of sight; let us now take one from the sense of touch. Suppose that, with your eyes shut, you let your finger-tip press against a hard table. What is really happening? The physicist says that your finger-tip and the table consist, roughly speaking, of vast numbers of electrons and protons; more correctly, each electron and proton is to be thought of as a collection of processes of radiation, but we can ignore this for our present purposes. Although you think you are touching the table, no electron or proton in your finger ever really touches an electron or proton in the table, because this would develop an infinite force. When you press, repulsions are set up between parts of your finger and parts of the table. If you try to press upon a liquid or a gas, there is room in it for the parts that are repelled to get away. But if you press a hard solid, the electrons and protons that try to get away, because electrical forces from your finger repel them, are unable to do so, because they are crowded close to others which elbow them back to more or less their original position, like people in a dense crowd. Therefore the more you press the more they repel your finger. The repulsion consists of electrical forces, which set up in the nerves a current whose nature is not very definitely known. This current runs into the brain, and there has effects which, so far as the physiologist is concerned, are almost wholly conjectural. But there is one effect which is not conjectural, and that is the sensation of touch. This effect, owing to physiological inference or perhaps to a reflex, is associated by us with the finger-tip. But the sensation is the same if, by artificial means, the parts of the nerve nearer the brain are suitably stimulated--_e.g._ if your hand has been amputated and the right nerves are skilfully manipulated. Thus our confidence that touch affords evidence of the existence of bodies at the place which we think is being touched is quite misplaced. As a rule we are right, but we can be wrong; there is nothing of the nature of an infallible revelation about the matter. And even in the most favorable case, the perception of touch is something very different from the mad dance of electrons