The reason is that, hitherto, no way has been discovered of doing without them. If we admit the spiritualistic theory, the discarnate spirits which surround us on every side and which are separated from us by the impenetrable and mysterious wall of death seek, in order to communicate with us, the line of least resistance between the two worlds and find it in the medium, without our knowing why, even as we do not know why an electric current passes along copper wire and is stopped by glass or porcelain. If, on the other hand, we admit the telepathic hypothesis, which is the more probable, we observe that the thoughts, intentions or suggestions transmitted are, in the majority of cases, not conveyed from one subconscious intelligence to another. There is need of an organism that is, at the same time, a receiver and a transmitter; and this organism is found in the medium. Why? Once more, we know absolutely nothing about it, even as we do not know why one body or combination of bodies is sensitive to concentric waves in wireless telegraphy, while another is not affected by it. We are here groping, as indeed we grope almost everywhere, in the obscure domain of undisputed but inexplicable facts. Those who care to possess more precise notions on the theory of mediumism will do well to read the admirable address delivered by Sir William Crookes, as president of the S.P.R., on the 29th of January 1897. [3] The questions of fraud and imposture are naturally the first that suggest themselves when we begin to study these phenomena. But the slightest acquaintance with the life, habits and proceedings of the three or four leading mediums is enough to remove even the faintest shadow of suspicion. Of all the explanations conceivable, that one which attributes everything to imposture and trickery is unquestionably the most extraordinary and the least probable. Moreover, by reading Richard Hodgson’s report entitled, _Observations of certain Phenomena of Trance_ (_Proceedings_, Vols. VIII. and XIII.) and also J. H. Hyslop’s report (_Proceedings_, Vol. XVI.), we can observe the precautions taken, even to the extent of employing special detectives, to make certain that Mrs. Piper, for instance, was unable, normally and humanly speaking, to have any knowledge of the facts which she revealed. I repeat, from the moment that one enters upon this study, all suspicions are dispelled without leaving a trace behind them; and we are soon convinced that the key to the riddle must not be sought in imposture. All the manifestations of the dumb, mysterious and oppressed personality that lies concealed in every one of us have to undergo the same ordeal in their turn; and those which relate to the divining-rod, to name no others, are at this moment passing through the same crisis of incredulity. Less than fifty years ago, the most of the hypnotic phenomena which are now scientifically classified were likewise looked upon as fraudulent. It seems that man is loth to