The metaphysical psychology of the present day, that has developed out of Descartes' theory of two substances absolutely different and yet externally joined together, this psychology seems unquestionably to be further away from the reality of the mental life than the theories of the ancient metaphysicians were. The old idea saw in the soul the principle of all life, or, according to Aristotle, the energy working towards an end, out of which the whole of the phenomena of life, physical and psychical, sprang. It sought at least to account for that unity of life, which popular dualism must regard as a wonder, if it does not suppose the psychical to be a confused image of the physical, or reversely suppose this latter to be a mere subjective idea without its own reality. And yet this old vitalistic idea of a soul is for us no longer possible. For it tries to explain the unity of life only by postulating an all-embracing idea of purpose or use in place of a causal explanation of phenomena such as is now demanded. This vague notion of purpose does not explain the peculiarity of mental processes, nor does it fulfil the requirements of a natural explanation in regard to the physical side of the phenomena of life. Nutrition, propagation, movement, on the one hand, and perception, imagination, understanding, on the other, cannot be combined into one unity, even although the facts which these concepts denote are purposeful from the standpoint of the connection of the phenomena of life. They do not resist such a combination because they are bound up with essentially different substrata, but because they depend upon absolutely different stand-points of the phenomena of life given to us as a unity. Nutrition, propagation, movement, are organic processes which belong to objective nature, and for which, because of their own characteristics, the ideas we form of them serve as signs which point to an existence independent of our consciousness. In investigating them, just as in the investigation of natural phenomena outside our own body, we must abstract from the subjective processes of consciousness, to which they are bound, if we wish to understand them in their objective natural connection. On the other hand our ideas, inasmuch as they are subjective, our feelings and our emotions are immediate experiences, which psychology tries to understand exactly in the way in which they arise, continue, and enter into relations with each other in consciousness. Therefore it is one and the same psycho-physical individual forming a unity, which physiology and psychology have as subject-matter. Each of these, however, views this subject-matter from a different stand-point. Physiology regards it as an object of external nature, belonging to the system of physical-chemical processes, of which organic life consists. Psychology regards it as the system of our experiences in consciousness. Now for every piece of knowledge two