One of the most interesting discoveries of physiology was the discovery, made simultaneously in France and Germany fifty years ago, that nerve currents do not only start muscles into action, but may check action already going on or keep it from occurring as it otherwise might. _Nerves of arrest_ were thus distinguished alongside of motor nerves. The pneumogastric nerve, for example, if stimulated, arrests the movements of the heart: the splanchnic nerve arrests those of the intestines, if already begun. But it soon appeared that this was too narrow a way of looking at the matter, and that arrest is not so much the specific function of certain nerves as a general function which any part of the nervous system may exert upon other parts under the appropriate conditions. The higher centres, for example, seem to exert a constant inhibitive influence on the excitability of those below. The reflexes of an animal with its hemispheres wholly or in part removed become exaggerated. You all know that common reflex in dogs, whereby, if you scratch the animal's side, the corresponding hind leg will begin to make scratching movements, usually in the air. Now in dogs with mutilated hemispheres this scratching reflex is so incessant that, as Goltz first described them, the hair gets all worn off their sides. In idiots, the functions of the hemispheres being largely in abeyance, the lower impulses, not inhibited, as they would be in normal human beings, often express themselves in most odious ways. You know also how any higher emotional tendency will quench a lower one. Fear arrests appetite, maternal love annuls fear, respect checks sensuality, and the like; and in the more subtile manifestations of the moral life, whenever an ideal stirring is suddenly quickened into intensity, it is as if the whole scale of values of our motives changed its equilibrium. The force of old temptations vanishes, and what a moment ago was impossible is now not only possible, but easy, because of their inhibition. This has been well called the 'expulsive power of the higher emotion.' It is easy to apply this notion of inhibition to the case of our ideational processes. I am lying in bed, for example, and think it is time to get up; but alongside of this thought there is present to my mind a realization of the extreme coldness of the morning and the pleasantness of the warm bed. In such a situation the motor consequences of the first idea are blocked; and I may remain for half an hour or more with the two ideas oscillating before me in a kind of deadlock, which is what we call the state of hesitation or deliberation. In a case like this the deliberation can be resolved and the decision reached in either of two ways:-- (1) I may forget for a moment the thermometric conditions, and then the idea of getting up will immediately discharge into act: I shall suddenly find that I have got up--or