I do not expect this book to stand upon its literary merits; for if it is unsound in principle, felicity of diction cannot save it, and if sound, homeliness of expression cannot destroy it. My primary object in offering it to the public is to assist in bringing Psychology within the domain of the exact sciences. That this has never been accomplished is owing to the fact that no successful attempt has been made to formulate a working hypothesis sufficiently comprehensive to embrace all psychic phenomena. It has, however, long been felt by the ablest thinkers of our time that all psychic manifestations of the human intellect, normal or abnormal, whether designated by the name of mesmerism, hypnotism, somnambulism, trance, spiritism, demonology, miracle, mental therapeutics, genius, or insanity, are in some way related; and consequently, that they are to be referred to some general principle or law, which, once understood, will simplify and correlate the whole subject-matter, and possibly remove it from the domain of the supernatural. The London Society for Psychical Research, whose ramifications extend all over the civilized world, was organized for the purpose of making a systematic search for that law. The Society numbers among its membership many of the ablest scientists now living. Its methods of investigation are purely scientific, and painstaking to the last degree, and its field embraces all psychic phenomena. It has already accumulated and verified a vast array of facts of the most transcendent interest and importance. In the mean time a large number of the ablest scientists of Europe and America have been pursuing independent investigations in the phenomena of hypnotism. They too have accumulated facts and discovered principles of vast importance, especially in the field of mental therapeutics, - principles which also throw a flood of light upon the general subject of Psychology.This vast array of facts, thus accumulated and verified, and awaiting scientific classification and analysis, would seem to justify at least a tentative effort to apply to them the processes of induction, to the end that the fundamental law of psychic phenomena may be discovered.In the following pages I have attempted such a classification of verified phenomena, accounts of which I find in the literature current on the subject; and I have tentatively formulated a working hypothesis for the systematic study of all classes of psychic phenomena. It will be observed that I have availed myself largely of the labors of others, instead of confining myself to experimental researches of my own. I have done this for two reasons: first, that I might avoid the accusation of having conducted a series of experiments for the purpose of sustaining a pet theory of my own; and second, because I hold that substantial progress cannot be made in science until one is ready to accord due credit to human integrity, and to give due weight to human testimony.In conclusion, I desire to say that I claim no credit for this work, save that which is due to an honest desire to promote the truth for its own sake. Sincerely believing in the correctness of my hypothesis, I have not hesitated to follow it to its legitimate conclusion in every field which I have entered. If at the close of the book I have seemed to trespass upon the forbidden field of theological discussion, it was not for the purpose of sustaining any preconceived opinions of my own; far from it. It was because I was irresistibly led to my conclusions by the terms of my hypothesis and the inflexible logic of its application. I cannot but be aware that my conclusions sometimes oppose the preconceived opinions of others. But no one who accepts my hypothesis as the true one will be compelled more frequently than I have been to renounce his former convictions.