ABNORMAL psychology, or the study of abnormal mental phenomena, is one of the late developments of scientific medicine. It is not a mere fad, as some of its critics would attempt to make us believe, neither has it sprung up like a mushroom, within a single night. Abnormal psychology is the outcome of the work of small groups of investigators in France, Germany, and America, within the last twenty- five years. Beginning with a study of the phenomena of hypnosis, these researches gradually developed into a series of brilliant psychological discoveries. The most important of these is the principle of dissociation or of N splitting of the mind. In a general way we speak of these matters as the theory of the subconscious. This theory has not only thrown an immense amount of light on the nature of human personality, but other peculiar phenomena, such as losses of memory or amnesia, automatic writing, crystal gazing, and such diseases as neurasthenia, hysteria, psychasthenia, have been stripped of the mystery which surrounded them for centuries. These phenomena, even more than the modern investigations on the ultimate nature of matter, form the "fairyland of science." Apart from any scientific knowledge, the general reader has a certain interest in these problems, either from curiosity or the light they shed upon human personality or perhaps from the mystery which seems to surround them. Abnormal psychology has also its practical aspects. Its discoveries have made possible the psychic treatment of certain functional nervous disorders. Technically, this is known as psychotherapeutics. The interpretation of these functional states is based on the principle of the dissociation of consciousness. But psychotherapeutics would be in a very chaotic condition and barren of results, were it not for abnormal psychology, for a scientific psychotherapeutics must be based upon a sound psychopathology.