In point of fact, the investigations of the Society for Psychical Research make it certain that in nine cases out of ten a poltergeist is a by-product of hysteria, using the term in its strictest medical sense. As is well known, one of the distinctive symptoms of hysteria is a tendency to indulge in all manner of lies and deceptions, coupled often with almost diabolical cleverness in giving these lies and deceptions a color of reality. Impulse to such trickery may arise from a great variety of motives; frequently, it would seem, from nothing more than an abnormal craving for notoriety and admiration. Certainly, the hysterical young people run to earth by the poltergeist hunters of the Society for Psychical Research did not engage in their hoaxings because they expected to make money out of them. The bearing of all this on the physical phenomena of spiritism is surely self-evident. It shows, for one thing, that the money motive is not the only motive inciting mediums to fraud; that when a neurotic or hysterical condition is present, the best of characters is no guarantee against duplicity; and that under such circumstances the detection of fraud is exceedingly difficult, particularly in the case of witnesses predisposed to regard the phenomena as genuine. If hysterical children can, as they have often done, carry on a course of deception mystifying a whole community, it is manifest that mediums of similar hysterical tendencies, working under cover of darkness or in a dim light, can more or less readily deceive the most expert observers; and, moreover, that they may be only partially, if at all, conscious of their own frauds.[31] [31] I am inclined, for example, to believe that there is a large element of hysteria in the mediumship of the discredited Eusapia Paladino, once the marvel of two continents. Further, in estimating the nature of the phenomena produced at the séances of physical mediums, it is imperative to take into account the innumerable possibilities of mal-observation on the part of the spectators. Experience has shown that comparatively few people, no matter how honest, are trustworthy witnesses even when conditions for observation are of the best. For proof of this, one does not need to look beyond the courtroom, where every day perfectly honest people give the most contradictory accounts of some simple occurrence. If it is thus difficult to see correctly what goes on in the broad light of day, it surely is far more difficult to be certain of exactly what is happening in a room where there is darkness rather than light. Besides which, the imaginative faculty may be excited to such an extent that the sitters at a séance may not only be misled into making inaccurate reports of what really occurred, but they may even, and with absolute sincerity, testify to phenomena which did not occur at all.