The history of human care for the insane, it is often said by those who are reviewing the whole subject briefly, may be represented by the steps in progress from the presumption of diabolical possession, and exorcism for its relief, to intelligent understanding, sympathetic treatment, and gentle surveillance, with the implication that this has all been a gradual evolution. There is no doubt that during the Middle Ages even physicians often thought of possession by the devil as the cause of irrational states of mind. Not only some of the genuinely insane--though not all, be it noted--but also sufferers from dreads and inhibitions of various kinds, the victims of tics and uncontrollable habits, especially the childish repetition of blasphemous words, and sufferers from other psychoses and neuroses, were considered to be the victims of diabolic action. Exorcism then became a favourite form of treatment of all these conditions, but its general acceptance came about because it was so often successful. The mental influence of the ceremonies of exorcism was often quite as efficient in the cure of these mental states as mesmerism, hypnotism, psycho-analysis, and other mental influences in the modern time. It may particularly be compared in this regard to psycho-analysis in our own day, for this cures patients by making them feel that they have been the victims of some very early evil impression, usually sexual in character, which has continued unconsciously to them to colour all their subsequent mental life. Some of the curious theories of secondary personality, the subliminal self and what has recently been called "our hidden guest," represent in other terms what the medieval observers and thinkers expressed in their way by an appeal to diabolic influence. They felt that there was a spirit influencing these patients quite independent of themselves in some way, and their thoroughgoing belief in a personal devil led them to think that there must be some such explanation of the phenomena. Even great scientists in the modern time who have studied psychic research have not been able to get away entirely from the feeling that there is something in such possession, and have admitted that there may be even alien influence by an evil spirit. The more one studies the question from all sides, and not merely from a narrow materialistic standpoint, the less one is ready to condemn the medievalists for their various theories of diabolic possession. The Christian Church still teaches not only its possibility but its actual occurrence.