the identification of the mind with a part of the working of the nervous system, need involve, theology apart, no disturbance of anyone’s attitude to the world, his fellow-men, or to himself. Theology, however, is still more implicit in current attitudes than traditional sceptics suspect. The nervous system is the means by which stimuli from the environment, or from within the body, result in appropriate behaviour. All mental events occur in the course of processes of adaptation, somewhere between a stimulus and a response. Thus every mental event has an origin in stimulation, a character, and consequences, in action or adjustment for action. Its character is sometimes accessible to introspection. What it feels like, in those cases in which it feels or is felt at all, is consciousness, but in many cases nothing is felt, the mental event is unconscious. Why some events are conscious but not others is at present a mystery; no one has yet succeeded in bringing the various hints which neurology may offer into connection with one another. In some important respects conscious and unconscious mental events must differ, but what these are no one can as yet safely conjecture. On the other hand there are many respects in which they are similar, and these are the respects which are at present most open to investigation. The process in the course of which a mental event may occur, a process apparently beginning in a stimulus and ending in an act, is what we have called an impulse. In actual experience single impulses of course never occur. Even the simplest human reflexes are very intricate bundles of mutually dependent impulses, and in any actual human behaviour the number of simultaneous and connected impulses occurring is beyond estimation. The simple impulse in fact is a limit, and the only impulses psychology is concerned with are complex. It is often convenient to speak as though simple impulses were in question, as when we speak of an impulse of hunger, or an impulse to laugh, but we must not forget how intricate all our activities are. To take the stimulus as a starting-point is in some ways misleading. Of the possible stimuli which we might at any moment receive, only a few actually take effect. Which are received and which impulses ensue depends upon which of our interests is active, upon the general set, that is, of our activities. This is conditioned in a large degree by the state, of satisfaction or unrest, of the recurrent and persistent needs of the body. When hungry and when replete we respond differently to the stimulus of a smell of cooking. A change in the wind unnoticed by the passengers causes the captain to reduce sail. Social needs in this respect are often as important as individual. Thus some people walking in a Gallery with friends before whom they wish to shine will actually receive far more stimulus from the pictures than they would if by themselves.