While many forms of mental disease have a physical origin in the brain, nervous system, and state of the blood, many others are purely mental from beginning to end, although the body may be chosen as the scene of some of their manifestations. Modern medicine is learning to deal with mental diseases by mental methods, and of these the principal types may be of interest. It must be remembered, however, that psychotherapy is the youngest of the sciences, and is still in its experimental stage; and that though magnificent work has been done by the pioneers, they cannot claim to have said the last word upon the structure of the human mind, for even if they knew all that was to be known, leaving nothing to be discovered by future investigation, which they would be the last to claim on their own behalf, though their disciples are not always blessed with the same modesty of genius, evolution is moving on, with the human mind at its apex, so that statements which were true of human nature before the Great War may have to be modified and supplemented when the Great Peace becomes an established fact.