The development of the idea of a soul in philosophy is in essentials a repetition of this mythological development. The ancient philosophy, in whose footsteps mediæval philosophy follows, still holds fast to the idea of a body-soul. The soul is the driving force of all, even physical processes of life, e.g. nutrition and propagation. By the side of this, however, the higher mental activities are bound to a specific being that is separable from the body. This opinion, which gave a concrete, clear form to the mythological ideas, found its most perfect scientific expression in the psychology of Aristotle. The psyche that was separable from the body had thus won a victory over the body-soul both in mythology and in the classical work of Aristotle. This, of course, led ultimately to the absolute dominion of this independent soul, and its qualities were more and more considered to be absolutely opposite to the qualities of the body, that was ruled by purely material laws. This development culminated in the system of Descartes, the last great philosopher of the Renaissance. The body is from now on considered to be an expended substance, subject to mechanical laws only; the soul stands, in contradistinction to this, as an unextended, purely thinking substance. The two substances are, however, during life externally joined together. In one single point of the brain the body was supposed to meet in reciprocal action with the soul, which was thought of as something analogous to a material atom. Descartes fixed upon the pineal gland, but there were countless other hypotheses as to the position of this point. This is not the place to follow the further changes that these ideas underwent in the history of modern philosophy and psychology. All the later changes of the dualistic hypothesis are not of the first importance. The fundamental principle is, that the soul is a permanent substance, and the psychical processes are looked upon as changing phenomena of this substance, which are, however, different from it. This hypothesis may take the form that Spinoza gave it in presupposing the two substances changed into two attributes which run parallel with each other. There is also the materialistic hypothesis that reduces the soul-substance to a quality of the bodily substance, which alone is recognised as real. It becomes clear to us that such further developments of the "substance" hypothesis become more and more contradictory to the laws of psychical life, the more they attempt to explain the self-contradicting conception of two absolutely different substances which must be bound together into one unity. The Cartesian soul can no longer exist in face of our present-day physiological knowledge of the physical substratum of our mental life. And metaphysical monism in these two forms, which try to combine soul-and body-substance into one unity, would shut out the possibility of any knowledge of our psychical life.