Unfortunately, the amount of psychology which is scientific and relevant to human nature is small. Much of it reduces to physiology. The chemistry of the glands and blood, the nature of sensation, feeling, emotion, and of the thinking processes, and those predispositions of the body which we call instincts are ultimately problems of physiology and await the development of that science for their solution. What distinguishes psychology from physiology is the study of behaviour in society: the transmutation of the instincts and temperament into habits and sentiments. Of psychology proper we have, so far, elaborate programmes, a bewildering variety of theories,--part epistemology, part logic, part metaphysics, and part mythology,--a vast accumulation of data, some classifications, and hardly any science. Yet the studies of “types,” “personality,” “culture forms,” and particularly the researches of psychiatrists contain many accurate descriptions of human behaviour. Their endeavour to explain the most sublimated actions, by reference to the constitution of the organism and to the determining factors of its environment, approaches sound science. But “behaviour in society” and “the determining factors of environment” are vague phrases. If there is to be a science of behaviour and society two conditions are necessary: first, that the organisms in spite of individual variations should be essentially the same; and second, that the environment to which they react should be stable and limited. For if each organism were essentially unique there would be no basis for the science of psychology, and if the environment were constantly changing there would be no conditions and no control for studying the organism. But human environment, the totality of forces which affect us and help satisfy our needs, is not stable and limited. It is plastic and relative. Tradition and convention are added to the forces composing an animal’s environment. These multiply the possibilities of behaviour indefinitely. In proportion as a man frees himself from a blind adherence to custom, the more personal his environment will be. To know such a man’s environment is to become acquainted with a unique world. It is to know not only his ancestry, his history, his habits, his occupation, and his social milieu, but also his purposes, the ends which he consciously seeks and which are the meaning of his actions to him. It is to know his mind. But a mind, being unique, can never be absolutely known. Even if it were completely revealed in behaviour, conversation, and writing, and if all the occasions that have served to form it were discovered, one could never be certain to capture a man’s understanding of his words, or his interpretation of his attitude and actions.