sources of error in human testimony, that it may be doubted whether we should be justified in attaching weight to the phenomena of telepathic hallucination and clairvoyance, to which a large part of this book is devoted, if the alleged observations were incapable of experimental verification. Certainly in such a case, though the recipient of an experience of this kind might cherish a private conviction of its significance, it would hardly be possible for such a view to win general assent. In fact, however, the clue to the interpretation of the more striking phenomena, in the case of which, since they occur for the most part spontaneously, direct experiment or even methodical and continuous observation are rarely possible, is furnished by actual experiment on a smaller scale and with mental affections of a less unusual kind. The thesis which these pages are designed to illustrate and support is briefly: _that communication is possible between mind and mind otherwise than through the known channels of the senses_. Proof of the existence of such communication, provisionally called _Thought Transference_ or _Telepathy_ (from _tele_ = at a distance, and _pathos_ = feeling), will be found in a considerable mass of experiments conducted during the last twelve years by various observers in different European countries and in America. Before proceeding, in the course of the next four chapters, to examine this part of the evidence in detail, it will be well to consider its various defects and sources of error--defects common in some degree to all experiments of which living beings are the subject, and sources of error for the most part peculiar to this and kindred inquiries. The word _experiment_ in this connection usually, and rightly, suggests the most perfect form of experiment, that in which all the conditions are known, and in which the results can be predicted both quantitatively and qualitatively. If, for instance, we add a certain quantity of nitric acid under given conditions to a certain quantity of benzine, we know that there will result a certain quantity of a third substance which is unlike either of its constituents in taste, smell, and physical properties. Or if we burn a given quantity of coal in a particular engine, we can predict, within narrow limits of error, the total amount of energy which will be evolved. That we cannot in the second instance predict with absolute accuracy the amount of energy produced is simply due to the difficulty of measuring with precision all the factors in the case. But when we leave the problems of chemistry and physics and approach the problems of biology, the difficulties increase a hundredfold. Here not only are we unable to measure the various factors, we cannot even name them. No skill or forethought would have enabled an observer, from however patient a study of parentage and environment, to have predicted the appearance, say, of Emanuel Swedenborg or Michael Faraday. Of the