The whole of what is called "modern occultism," including spiritualism, second-sight, thought transference (so-called telepathy), crystal-gazing, astrology, and such mysteries, can only be treated reasonably in the way I have mentioned. We ask for a demonstration of the occurrence of the mysterious communications or prophecies, or "raps" or "levitations," or whatever it may be. Lovers of science have never been unwilling to investigate such marvels if fairly and squarely brought before them. In the very few cases which have been submitted in this way to scientific examination, the marvel has been shown to be either childish fraud or a mere conjurer's trick, or else the facts adduced in evidence have proved to be entirely insufficient to support the conclusion that there is anything unusual at work, or beyond the experience of scientific investigators. It is unfortunately true that most persons are quite unprepared to admit the deficiencies of their own powers of observation and of memory, and are also unaware of their own ignorance of perfectly natural occurrences which continually lead to self-deception and illusion. Moreover, the capacity for logical inference and argument is not common. The whole past and present history of what is called "the occult" is enveloped in an atmosphere of self-deception and of readiness to be deceived by others to which misplaced confidence in their own cleverness and power of detecting trickery renders many--one may almost say most--people victims. The physician who has given his life to the study of mental aberration and diseases of the mind is the only really qualified investigator of these "marvels," and no one who has closely studied what is known in the domain of mental physiology and pathology has any difficulty in understanding, and bringing into relation with large classes of established facts as to illusions and mental aberration, the "beliefs" in magic and second-sight which are here and there found flourishing at the present day, as well as the, at first sight startling, evidence of highly accomplished men who have suffered from such delusions.