Before proceeding, in the chapters which follow, to cite instances of hallucinations which purport to have been telepathically originated, it seems needful to glance briefly at sensory hallucination in general. To most persons, no doubt, the word connotes disease. Their ideas of hallucination are probably derived from vague reports of asylum experience and _delirium tremens;_ or at least from the cases of Goethe's butt, Nicolai, the Berlin bookseller, and the Mrs. A. whose experiences are described in Brewster's _Letters on Natural Magic_, both of whom are known to have been under medical treatment for illness of which the hallucinations were regarded as a symptom. Indeed, until recent years the tendency of even well-instructed opinion has been to regard a sensory hallucination as necessarily implying some physical or mental disorder. This misconception--for it is a misconception--has had some curious consequences. Since it does occasionally happen that a person admittedly sane and healthy reports to have seen the likeness of a human figure in what was apparently empty space, such reports have been by some perforce scouted as unworthy of credence, and by others regarded as necessarily indicating some occult cause--as testifying, in short, to the agency of "ghosts." There was indeed the analogy of dreams to guide us. Few educated persons would regard dreams, on the one hand, as a symptom of ill-health, or on the other as counterparts or revelations of any super-terrestrial world; or, indeed, as anything else than purely subjective mental images. Yet dreams belong to the same order of mental phenomena as hallucinations, and are commonly so classed--such differences as exist being mainly due to the conditions under which the two sets of phenomena respectively occur. In fact, a hallucination is simply a hypertrophied thought--the last member of a series, whose intermediate terms are to be found in the mind's-eye pictures of ordinary life, in the vivid images which some artists can summon at will, and in the Faces in the Dark which many persons see before passing into sleep, with its more familiar and abundant imagery.