The book which is now at last given to the world is but a partial presentation of an ever-growing subject which I have long hoped to become able to treat in more adequate fashion. But as knowledge increases life rolls by, and I have thought it well to bring out while I can even this most imperfect text-book to a branch of research whose novelty and strangeness call urgently for some provisional systematisation, which, by suggesting fresh inquiries and producing further accumulation of evidence, may tend as speedily as possible to its own supersession. Few critics of this book can, I think, be more fully conscious than its author of its defects and its lacunæ; but also few critics, I think, have yet realised the importance of the new facts which in some fashion the book does actually present. Many of these facts have already appeared in _Phantasms of the Living_; many more in the _Proceedings_ of the Society for Psychical Research; but they are far indeed from having yet entered into the scientific consciousness of the age. In future years the wonder, I think, will be that their announcement was so largely left to a writer with leisure so scanty, and with scientific equipment so incomplete. Whatever value this book may possess is in great measure due to other minds than its actual author's. Its very existence, in the first place, probably depends upon the existence of the two beloved friends and invaluable coadjutors to whose memory I dedicate it now. The help derived from these departed colleagues, Henry Sidgwick and Edmund Gurney, although of a kind and quantity absolutely essential to the existence of this work, is not easy to define in all its fulness under the changed circumstances of to-day. There was indeed much which is measurable;--much of revision of previous work of my own, of collaborative experiments, of original thought and discovery. Large quotations purposely introduced from Edmund Gurney indicate, although imperfectly, how closely interwoven our work on all these subjects continued to be until his death. But the benefit which I drew from the association went deeper still. The conditions under which this inquiry was undertaken were such as to emphasise the need of some intimate moral support. A recluse, perhaps, or an eccentric,--or a man living mainly with his intellectual inferiors, may find it easy to work steadily and confidently at a task which he knows that the bulk of educated men will ignore or despise. But this is more difficult for a man who feels manifold links with his kind, a man whose desire it is to live among minds equal or superior to his own. It is hard, I say, for such a man to disregard altogether the expressed or implied disapproval of those groups of weighty personages to whom in other matters he is accustomed to look up.