There are a number of sciences which deal with Man. We may deal with him in natural history, as one among the animals, having a certain place in evolution, and related to other animals in ascertainable ways. We may deal with him in physiology, as a structure capable of performing certain functions, and reacting to the environment in ways of which some, at least, can be explained by chemistry. We may study him in sociology, as a unit in various organisms, such as the family and the state. And we may study him, in psychology, as he appears to himself. This last gives what we may call an internal view of man, as opposed to the other three, which give an external view. That is to say, in psychology we use data which can only be obtained when the observer and the observed are the same person, whereas in the other ways of studying Man all our data can be obtained by observing other people. There are different ways of interpreting this distinction, and different views of its importance, but there can be no doubt that there is such a distinction. We can remember our own dreams, whereas we cannot know the dreams of others unless they tell us about them. We know when we have toothache, when our food tastes too salty, when we are remembering some past occurrence, and so on. All these events in our lives other people cannot know in the same direct way. In this sense, we all have an inner life, open to our own inspection but to no one else’s. This is no doubt the source of the traditional distinction of mind and body: the body was supposed to be that part of us which others could observe, and the mind that part which was private to ourselves. The importance of the distinction has been called in question in recent times, and I do not myself believe that it has any fundamental philosophical significance. But historically it has played a dominant part in determining the conceptions from which men set out when they began to philosophise, and on this account, if on no other, it deserves to be borne in mind.