All scientific explanation consists in reducing the unknown to terms of the known. We can often _classify_ a phenomenon without being able to _explain_ its innermost nature. If we discover its laws, we have advanced to that extent. Dr. J. Ochorowicz, who has made a prolonged and minute study of this question, writes as follows regarding the necessary conditions to be observed in the operator: "On the side of the _operator_ the conditions have been very little studied. But it is probable: "1. That there are personal differences. "2. That these differences may be due not only to the degree of thought intensity, but also to the nature of the thought itself, according as it is visual, auditive, or motor. "3. That some account has to be taken of a sort of accord, of concordance between the two intelligences. "4. That excessive will-power impairs the definiteness of the transmission without much enhancing its intensity. "5. That strong, persistent, prolonged thinking of a thought repeated for a longer or shorter time constitutes a condition in the highest degree favourable. "6. That any distraction which causes the thought to disappear for a moment, or that makes it cease to be isolated, seems eminently unfavourable to the mental action. "7. That, nevertheless, thoughts that are not intense, and even thoughts that are at the moment unconscious (subconscious), may be transmitted involuntarily. "8. That the muscular efforts which usually accompany an exertion of will are more or less indifferent; but that the muscle expression of the operator may be useful, subjectively, by reason of the habitude that connects thought with these expressional signs. "It follows from these considerations that the operator should insist less upon the 'I will it' than upon the content of that willing; and hence it is probable that, properly speaking, it is not the 'strong will' that helps telepathy so much as clear thinking." As to the subject or _percipient_, experience has taught us that the four following states are probably the most important for the recipience of a telepathic message: 1. In the state of profound _aideia_ (complete lack of thought) transmission is never immediate, but it may sometimes be latent. 2. In the state of nascent _monoideism_ (one idea) it may be immediate and perfect. 3. In the state of _passive polyideism_ (many thoughts) it may be either immediate or may take place after an interval of greater or lesser length. 4. In the state of _active polyideism_ the conditions are complex and subject to further subdivisions, for: (_a_) Transmission may be direct if the subject helps by voluntary self-absorption in a concentration of mind more or less monoideic; he lends himself to the action; he listens mentally; he seeks, sometimes he finds!