Than this disease—of which most people, unfortunately, have next to no exact knowledge, mistakenly confusing it with, and confining it to, uncontrollable attacks of weeping or laughing—there is no malady more insidious, peculiar, or dangerous in the variety of its possible consequences. Its peculiarity lies in the fact—discovered only within recent years—that it is always rooted in an extreme “suggestibility” on the part of its victims; and that the symptoms it develops are invariably conditioned by the character of the suggestions received from the environment. Hysteria is, to put the case briefly, pre-eminently a mental trouble; and this although, not infrequently, its only outward manifestations are wholly physical. A child with a hysterical tendency—that is to say, an unusually sensitive, impressionable child, of undisciplined will, and quickly overwhelmed by whatever it sees, hears, or feels—is always liable, when brought into contact with a person suffering from any serious ailment of picturesque symptomatology, to manifest in some degree the symptoms of that particular ailment. Or, more commonly, such a child may manifest grave physical disabilities simply as a result of hearing or reading about them. It does not do this voluntarily; there is no conscious intention to deceive; for the matter of that, the child itself is as much deceived as are its parents and friends. The trouble is that in its state of abnormal suggestibility, it is irresistibly impelled by the strange power of self-suggestion to imitate the symptoms of disease. Or, instead of simulating disease symptoms, a hysterical child may enter on a course of seemingly deliberate chicanery like that practised by little “poltergeist” Polly Turner, whose case is typical of a species of behaviour indulged in by hysterical children in all countries and all ages. Here, likewise, abnormal suggestibility is in evidence, the resultant hysterical manifestations differing only because the suggestions received and acted on are different. In cases like Polly Turner’s, it has been found, the hysterical child usually lives with people more or less superstitious and credulous. They are people inclined to attribute to some spiritistic agency any occurrence they cannot easily explain. In this environment the child gradually becomes obsessed—though quite unconsciously—with a desire to provide “marvels” for their edification and mystification, and, yielding to the desire, is soon in full career as a “poltergeist,” the hysterical obsession becoming intensified in proportion as the gullibility of those deceived increases, and also in proportion to the amount of attention paid to the little deceiver.