Considering the instincts from the standpoint of evolution, one may assume that they first developed in extremely low forms of life in order to produce the few and simple reactions of which animals low in the scale are capable. One might almost say in regard to such primitive organisms, that for each situation an instinct is provided and the situation calls forth its appropriate reaction almost as automatically as the pressing of an electric button causes the ringing of a bell. But, as animals rise higher in the scale, the kinds of conduct required become more varied and complex. For example, an impulse from the flight or fear instinct, in the lower animals, will always produce some simple reaction such as flight or concealment. But, in man, the forms of conduct, to which it gives rise, may be extremely varied. Thus in one case a man may be impelled to run away, in another to work hard at some disagreeable task in order to escape the harm which might result if he failed to do so. This capacity to direct the instinctive forces into various forms of activity, we call the capacity for adjustment and we may assume that it depends upon the operation of certain mechanisms which we may call the mechanisms of adjustment. The mind may, therefore, be regarded as made up of certain instincts from which come the impulses that give rise to all our mental reactions and certain mechanisms of adjustment by which these impulses are directed into the most useful forms of activity. This conception of the human mind enables us to form some idea of how a mental disorder may arise from purely mental causes; for it is obvious that conditions may sometimes arise when the mechanisms of adjustment will prove inadequate to the demands made upon them, when they will be unable to control the instinctive forces or find for them satisfactory outlet and, as a result, these impulses will escape by undesirable channels, giving rise to forms of thought and action which we recognize to be abnormal. To show that this theory may be successfully applied to explain the facts of abnormal psychology, the analysis of an illustrative case is presented. This case, which is worked out in considerable detail, forms the second section of the paper. It is the case of a young man who, partly owing to inherited tendencies and partly to environment, developed during early life certain habits and characteristics which, when he approached maturity and the sexual instinct awoke to its full activity, caused the impulses from this instinct to be directed into wrong channels, giving rise to a psychosis which took the form of a catatonic stupor.