In a letter published in _Mind_ (April 1888) Professor Royce, of Harvard, U.S.A., hazarded a hypothesis that there may occur "instantaneous and irresistible hallucinations of memory which make it seem to one that something which now excites or astonishes him has been prefigured in a recent dream, or in the form of some other warning." In support of that hypothesis Professor Royce appeals to the analogy of the well-known cases of double memory,--the impression of having at some previous time looked on a scene now present, or heard a conversation now taking place; and to two or three instances of undoubted hallucination of memory amongst the insane, recorded by Krafft-Ebing and Kraepelin. As regards the latter, it is sufficient to remark that the hallucinations occurred to persons whose minds were admittedly diseased; that the hallucinations themselves were apparently slow of growth, whereas the hypothesis requires that they should be more or less instantaneous; and that in other respects they do not present by any means a perfect parallel to the presumably telepathic cases with which he compares them. In default, therefore, of more precise analogies, the hypothesis of pseudo-presentiment must be regarded as, at best, a plausible guess. And even if it were fully substantiated it would only, as pointed out by Mr. Gurney (_Mind_, July 1888), apply to certain classes of telepathic cases, and those the weakest from the evidential standpoint. At most the theory would account for dreams and indefinite impressions of various kinds not mentioned beforehand. In some cases of this kind, and in a large class of so-called "prophetic" dreams, I am inclined to regard Mr. Royce's explanation as possibly true, in the modified form suggested by Dr. Hodgson (_Proc. American S.P.R._, pp. 540 _et seq._)--_i.e._, if it is restricted to cases where there is a vague memory of some actual dream or other impression, bearing a more or less remote resemblance to the event; in other words, if we assume an illusion rather than a hallucination of memory. But it need hardly be said that no serious investigator would treat the uncorroborated accounts of dreams and vague feelings of this kind as evidence for anything whatever. To extend the hypothesis, as Professor Royce suggests, to cases where there is evidence that the percipient's experience was mentioned beforehand, is to suppose not one kind of pseudo-memory, but two,--a pseudo-memory on the part of the percipient that he has had a certain subjective experience, and a pseudo-memory on the part of some other person that this experience was mentioned to him before the news of the event to which it related. In recent cases, at any rate, the assumption of a double mistake of this kind seems unwarranted.[78] And to apply this explanation to cases of actual sense-hallucination involves even more violent improbabilities. It would require far more evidence than