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 draught 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.
Vital movements have a stimulus which may be inside or outside the body, or both at once. Hunger is a stimulus inside the body, but hunger combined with the sight of good food is a double stimulus, both internal and external. The effect of a stimulus may be, in theory, according to the laws of physics and chemistry, but in most cases this is, at present, no more than a pious opinion. What we know from observation is that behaviour is modified by experience, that is to say, that if similar stimuli are repeated at intervals they produce gradually changing reactions. When a bus conductor says "Fares, please", a very young child has no reaction, an older child gradually learns to look for pennies, and, if a male, ultimately acquires the power of producing the requisite sum on demand without conscious effort. The
