In the preceding chapter we have collected together certain of the more prominent phenomena of mental disorder. To the uninitiated eye they appear as a chaos of disconnected facts, each seemingly independent of all the rest. A careful examination will show, however, that there are certain general attributes common to many of the symptoms displayed by our patients. With the aid of these general attributes it is possible to classify the phenomena into groups, and thereby to reduce the confusion to some sort of order.

Firstly, the conditions described as mental deficiency and dementia may be placed in a separate group. They depend, as we have seen, upon an absolute quantitative defect of mental capacity, and are in this respect fundamentally different from all the other phenomena dealt with in chapter III. These conditions are generally accompanied by a correlated defect in the structure of the brain, and it must be frankly confessed that, in the present state of knowledge, the psychological conception is of relatively little value in the study of their causation, and that the physiological conception is a far more profitable instrument of investigation. It is true, of course, that psychology is compelled to take account of such quantitative defects of mental capacity, and that it is able to explain the relation which they bear to many of the other phenomena of insanity, but the questions involved are complicated and difficult, and cannot be satisfactorily considered without presupposing a considerable practical acquaintance with the subject.