At the very threshold of this difficult province we encounter the curious fact that there are two radically distinct tendencies in psychology at our universities to-day. On one side we have the metaphysical and professional psychologists. They still cling to the older view that man's soul is a special entity, a unique independent individuality, which dwells for a time only in the mortal frame, leaving it and living on as an immortal spirit after death. This dualistic theory is connected with the doctrine of most religions, and owes its high authority to the fact that it is associated with the most important ethical, social, and practical interests. Plato gave prominence to the idea of the immortality of the soul in philosophy long ago. Descartes at a later date gave emphasis to it by ascribing a true soul to man alone and refusing it to the animals. This metaphysical psychology, which ruled alone for a considerable period, began to be opposed in the eighteenth, and still more in the nineteenth, century by _comparative psychology_. An impartial comparison of the psychic processes in the higher and lower animals proved that there were numerous transitions and gradations. A long series of intermediate stages connects the psychic life of the higher animals with that of man on the one side, and that of the lower animals on the other. There was no such thing as a sharp dividing line, as Descartes supposed. But the greatest blow was dealt at the predominant metaphysical conception of the life of the soul thirty years ago by the new methods of _psychophysics_. By means of a series of able experiments the physiologists, Theodor Fechner and Ernst Heinrich Weber of Leipsic, showed that an important part of the mental activity can be measured and expressed in mathematical formulæ just as well as other physiological processes, such as muscular contractions. Thus the laws of physics control a part of the life of the soul just as absolutely as they do the phenomena of inorganic nature. It is true that psychophysics has only partially realised the very high expectations that were entertained in regard to its Monistic significance; but the fact remains that a part of the mental life is just as unconditionally ruled by physical laws as any other natural phenomena.