The classification of diseases was carried out at a time when the body was regarded as the whole of man and the mind looked upon as an unimportant by-product whose influence was negligible. Modern discovery, however, has radically changed our outlook. Much mental disease has a physical origin and should not be classified as mental at all. Setting aside this type of disease, with which psychology, strictly speaking, is not concerned, we find the true mental diseases fall into a first broad division, those which are congenital and those which are acquired. In congenital disease an abnormal individual breaks down in a normal environment, and in acquired disease a normal individual breaks down in an abnormal environment. The boundary line between a healthy and diseased mind is not easy to draw, but we may reckon a mind diseased when it fails to react normally to its environment.