A few words must be said about the human body as a mechanism. It is an inconceivably complicated mechanism, and some men of science think that it is not explicable in terms of physics and chemistry, but is regulated by some “vital principle” which makes its laws different from those of dead matter. These men are called “vitalists”. I do not myself see any reason to accept their view, but at the same time our knowledge is not sufficient to enable us to reject it definitely. What we can say is that their case is not proved, and that the opposite view is, scientifically, a more fruitful working hypothesis. It is better to look for physical and chemical explanations where we can, since we know of many processes in the human body which can be accounted for in this way, and of none which _certainly_ cannot. To invoke a “vital principle” is to give an excuse for laziness, when perhaps more diligent research would have enabled us to do without it. I shall therefore assume, as a working hypothesis, that the human body acts according to the same laws of physics and chemistry as those which govern dead matter, and that it differs from dead matter, not by its laws, but by the extraordinary complexity of its structure. The movements of the human body may, none the less, be divided into two classes, which we may call respectively “mechanical” and “vital”. As an example of the former, I should give the movement of a man falling from a cliff into the sea. To explain this, in its broad features, it is not necessary to take account of the fact that the man is alive; his centre of gravity moves exactly as that of a stone would move. But when a man climbs up a cliff, he does something that dead matter of the same shape and weight would never do; this is a “vital” movement. There is in the human body a lot of stored chemical energy in more or less unstable equilibrium; a very small stimulus can release this energy, and cause a considerable amount of bodily movement. The situation is analogous to that of a large rock delicately balanced on the top of a conical mountain; a tiny shove may send it thundering down into the valley, in one direction or another according to the direction of the shove. So if you say to a man “your house is on fire”, he will start running; although the stimulus contained very little energy, his expenditure of energy may be tremendous. He increases the available energy by panting, which makes his body burn up faster and increases the energy due to combustion; this is just like opening the draft in a furnace. “Vital” movements are those that use up this energy which is in unstable equilibrium. It is they alone that concern the bio-chemist, the physiologist, and the psychologist. The others, being just like the movements of dead matter, may be ignored when we are specially concerned with the study of Man.