This little book will in one respect be of special interest to psychologists. The author confesses in the preface that when he commenced his hypnotic investigations, he attempted to explain the facts under consideration by the Cartesian theory which hitherto, he says, had proved perfectly sufficient to explain the data of normal soul-life. What the author understands by the Cartesian theory appears from the following passage: "The popular conception of the relation between soul and body is, that the soul is a being distinct from the body and endowed with certain faculties. This conception is still defended by a certain, not very numerous school of philosophers whom we may briefly call Cartesians from the fact that their theory can be traced back to Descartes, although in the lapse of time it has been considerably modified." In a word the Cartesian theory is the theory that still accepts the existence of a mythical or metaphysical soul-unity called the ego. Dr. Lehmann says: "It was argued since 'I' in spite of a constant change of my consciousness, am in possession of the certainty that it is the same identical 'I' that has all these states, sensations, feelings, this 'I' or the soul must be a unity. And this unity must stand in a causal connection with the outside world, with the domain of nature in the widest sense of the word," etc. It is perhaps exceptional that a teacher at a University of Protestant Northern Europe has been under the influence of Cartesianism, but it is highly commendable that he openly confesses his change of opinion because the facts under observation demonstrate its erroneousness. Dr. Lehmann no doubt will find that the normal phenomena of psychic life are by no means in accord with the Cartesian doctrine. Indeed by showing how the abnormal and normal states agree, he implicitly confesses that the theory that proves untenable for the former ought to be regarded as untenable for the latter. We have instances of men who believe in the Cartesian doctrine, or at least by a natural predisposition have a tendency to believe in it, wavering in their belief, because the data of the normal states of psychic life so little favor the dualism of the great French philosopher. Now it almost seems as if the discoveries and the strangeness of hypnotic phenomena had contributed a great deal towards turning the tendency toward a monistic solution of the psychological problems back to the almost abandoned dualistic solution. We are fully confident that this reaction will not last, because in spite of all the strange mysteries that surround modern hypnotism, it will after all only find a satisfactory interpretation in some monistic conception.