"My third theory dispenses with the agency of disembodied spirits, but involves us in other and perhaps equally great improbabilities. It is that the first appearance is a purely subjective hallucination, and that the subsequent similar appearances, both to the original percipient and to others, are the result of the first appearance; unconscious expectancy causing them in the case of the original percipient, and some sort of telepathic communication from the original percipient in the case of others. In fact, it assumes that a tendency to a particular hallucination is in a way infectious. If this theory be true, I should expect to find that the apparently independent appearances after the first depended on the percipient's having had some sort of intercourse with some one who had seen the ghost before, and that any decided discontinuity of occupancy would stop the haunting. I should also expect to find, as we do in one of the cases I have quoted, that sometimes the supposed ghost would follow the family from one abode to another, appearing to haunt them rather than any particular house. "The fourth theory that I shall mention is one which I can hardly expect to appear plausible, and which, therefore, I only introduce because I think that it corresponds best to a certain part of the evidence;--and, as I have already said, considering the altogether tentative way in which we are inevitably dealing with this obscure subject, it is as well to express definitely every hypothesis which an impartial consideration of the facts suggests. It is that there is something in the actual building itself--some subtle physical influence--which produces in the brain that effect which, in its turn, becomes the cause of a hallucination. It is certainly difficult on this hypothesis alone to suppose that the hallucinations of different people would be similar, but we might account for this by a combination of this hypothesis and the last. The idea is suggested by the case, of which I have given an abstract, where the haunting continued through more than one occupancy, but changed its character; and if there be any truth in the theory, I should expect in time to obtain a good deal more evidence of this kind, combined with evidence that the same persons do not as a rule encounter ghosts elsewhere. I should also expect evidence to be forthcoming supporting the popular idea that repairs and alterations of the building sometimes cause the haunting to cease."[159] These hypotheses--none of which, as Mrs. Sidgwick expressly states (_op. cit._, p. 145), seemed to herself satisfactory--did nevertheless, I think, comprise all the deductions which could reasonably be made from the evidence as it at that time stood. A few modifications, which the experience of subsequent years has led me to introduce, can hardly be said to afford further _explanation_, although they state the difficulties in what now seems to me a more hopeful way.