It is important, at the outset, to define the purposes which can be served by studying the subject of neurosis in animals, and they appear to be twofold. Firstly, animals display emotions in a simplified form, because their behaviour is not influenced by social conventions, and thus one can sometimes recognize vital underlying human feelings which are masked by other less important habits. A simple example of this is seen when two animals are given identical meals in separate feeding dishes; each animal is convinced that the other has a better meal, and they will not be happy till they have changed dishes: in human relationships, the feeling that the other person has something better often plays a major part in determining behaviour, but it is frequently disguised and seldom admitted by the person concerned. The second way in which animals can help in the study of human medicine is in controlled experimental work; for instance, the relative effects of heredity and environment can be studied comparatively easily in animals, and although it is important to remember that arguments based on animal behaviour cannot be applied directly to human behaviour, much can be gained by an exchange of information between workers in the two fields.