The rational soul, the soul which thinks, that is, understands, wishes, imagines, remembers, and feels, is not material. Yet it always acts through the machine, though that machine can go on perfectly well without the soul. ‘When the body has all its organs properly arranged for a particular movement it has no need of the soul to carry them out. All movements, even those which we call voluntary, depend principally on the same disposition of the organs. One and the same cause renders the dead body unfit to produce the movements and leads the soul to quit the body.’ The biology of Descartes appears to have been accepted by contemporary physiologists like van Helmont and Borelli, and certainly commended itself to another philosopher of eminence, Leibnitz. Like Descartes, Leibnitz also affirms that the body is a machine or natural automaton; unlike Descartes, however, he refuses to believe that the mind directs the machine in any way. Rather there is a complete series of psychical parallel to a complete series of physical events, and between the two a pre-established harmony. Although the details of Cartesian physiology have long since been exploded, yet the mechanical principle which that philosophy enunciated so clearly has persisted and has indeed proved to be the rock on which modern physiological science has been built. For, when once the chemists had discovered animal and plant structure to be composed of elements found in lifeless bodies, and had proved that compounds found only in the organism could yet be synthesized _in vitro_, there was no longer any reason why the properties of the compounds should be considered as of a different order to the properties of their component elements. A method applicable to one was applicable to the other, and as Claude Bernard has put it, mechanical, physical, and chemical forces are the only effective agents in the living body, and they are the only agencies of which the physiologist has to take account. The substances of which the living body is made up are no doubt extremely complex, yet none the less--to quote a more recent writer, Verworn--‘physiology is in the last resort the chemistry of the proteids’. This is the principle that has now for nearly a century guided and stimulated research into the functions of the organism: to this principle physiologists, too numerous to name, have not been ashamed to subscribe: under its banner some of the proudest triumphs of the science have been won. Yet it is precisely this which modern or neo-vitalism has challenged and asks us to relinquish in favour of a theory of psychoids or a pseudo-metaphysical view of life. The vitalistic position may be assailed from two points, the scientific and the philosophical. In the first place the vitalist asserts that mechanism is inadequate to explain the phenomena of metabolism, of transmission of nervous stimuli, or of development. It is upon the last of these that Driesch lays special stress.