Much mental disease has a physical origin and should not be classified as mental at all. To this class belong the mental disturbances arising from disease of or injury to the brain; womb trouble; poisoned blood conditions and the faulty functioning of the ductless glands, whose place in our economy is so important and so little understood; and many other causes of a like nature. 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. In both cases the results are the same, but treatment and prospect of recovery are very different. 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; thus, if happenings which should stir us deeply leave us unmoved, or we are upset by things which should have no power to disturb us, we may consider our mind is not working well. Let it never be forgotten, however, that mental disturbance ranges from irritability, depression, and bad memory, to its extreme manifestations in the different forms of insanity. The division between nervous and mental disease is even harder to draw, but for all practical purposes the sense of reality may be utilised as a dividing line; as soon as he loses his sense of reality a man passes the boundary line of insanity. The neurotic knows that there is something wrong with him, but that the world is all right; the lunatic believes that he is all right, but that there is something wrong with the world. It is the constant aim of the mind to maintain harmonious relations between the individual and the environment; to secure an adjustment to, and to make the best of, the constantly varying conditions to which the organism is subjected. If it fails to do this, the law of the survival of the fittest comes into action and automatically eliminates the unfit--those who have failed to adapt themselves to the conditions in which they live. Failure to adapt may be due to one of two causes: the individual may be abnormal, or the environment may be abnormal. Modern social conditions in a civilised community tend to prevent the automatic elimination of the unfit and to permit them to live on. With physical failure to adapt, due to malformation or lack of stamina, we will not deal here, but will confine ourselves to the problem of adjustment on the mental level.