23. It must not be thought that this "possession" theory of the Piper and similar cases is the only one which has been held in the past. On the contrary, as we know, there have been several others--Mrs. Sidgwick's telepathic theory--from the discarnate; Mr. Andrew Lang's theory of telepathy _a trois_; Mr. Podmore's theory of simple telepathy; the theory held by Andrew Jackson Davis and other clairvoyants, that there exists a sort of mirror-like sphere, upon which all thoughts and acts are recorded, and which the medium is somehow enabled to "read" during the trance state; the theory that discarnate spirits somehow project their thoughts upon a wax-like surface of astral substance, and that the medium is enabled to reinterpret them in some mysterious manner; the Theosophical theory; the theory of the occultists and mystics; the Catholic theory--that these manifestations are all the result of evil, lying spirits--these are but a few of the hypotheses which have been advanced in the past by way of explanation of these phenomena. I may say that this latter theory has some respectable evidence in its support, by the way, a few very remarkable cases having come under my own observation, which I hope to detail at some future time; and Dr. J. Godfrey Raupert has cited some impressive cases in his _Dangers of Spiritualism_, _Modern Spiritism_, and _The Supreme Problem_. This is assuredly a side of psychic investigation which demands close study and prolonged investigation; and, in spite of the masterly analysis of some of these cases by Professor Flournoy in his _Spiritism and Psychology_ (chap. iii.), I cannot but feel that there is yet much to be learned as to the nature of the intelligence manifested in these cases. And this was, as we know, the opinion also of Professor William James, for he wrote (_Proceedings of S.P.R._, vol. xxiii. p. 118): "The refusal of modern 'enlightenment' to treat 'possession' as a hypothesis to be spoken of as even possible, in spite of the massive human tradition based on concrete experience in its favour, has always seemed to me a curious example of the power of fashion in things scientific. That the demon theory (not necessarily a devil theory) will have its innings again is to my mind absolutely certain.... One must be blind and ignorant indeed to suspect no such possibility...." It must by no means be taken for granted, therefore, that the intelligences operating through Mrs. Piper and other mediums are all that they claim to be, even if their externality to the medium were proved.... We must be extremely cautious in accepting any messages coming through mediums until the most certain and convincing proofs of identity be forthcoming--and _then_ we should be cautious!