Another advocate of the doctrine of a vital force, a property of the tissues of the body, and at perpetual war with those inorganic forms which tend to their destruction, was the physiologist Bichat, Such a conception as this could not of course survive the rise of modern chemistry. Its death-knell was sounded when Lavoisier and Laplace showed that the bodies of organisms were composed of the same elements as are found in inanimate nature, and it has long since passed into the limbo of discredited speculations. Apart from this, vitalistic theories would appear to be in the main of two kinds. First, there is the metaphysical vitalism which tells us we can never explain the living in terms of the lifeless, insists on the permanent separation of the sciences of biology on the one hand from chemistry and physics on the other, and preaches the autonomy of the organism without venturing to tell us in what that autonomy consists. Secondly, there is the psychological theory of animism which posits an autonomous psychical entity to preside over the chemical and mechanical operations of the body, whether already formed or in process of development, and to direct them towards its own ends, the conservation and reproduction of that body’s specific form. A third party, halting between two opinions, suggests an unconscious idea, without, however, clearly explaining whether this is to be taken in a metaphysical or a psychological sense. Frankly opposed to vitalism in all its forms is the conception of the living body as a mechanism. This has also an honourable ancestry behind it. How far the biology of Aristotle is to be looked upon as mechanistic we shall presently have to inquire, but in Galen the soul is certainly material, or quasi-material, as we have already observed. It is, however, in the physiology of Descartes that mechanism first appears unmistakably in its modern guise. For Descartes the body is simply an earthly machine. The nerves are tubes up which--in sensation--the animal spirits flow to the brain only to be reflected (whence our term reflex action) down other tubes to the muscles. ‘All the functions of the body’, he tells us, ‘follow naturally from the sole disposition of its organs, just in the same way that the movements of a clock or other self-acting machine or automaton follow from the arrangement of its weights and wheels. So that there is no reason on account of its functions to conceive that there exists in the body any soul, whether vegetative or sensitive, or any principle of movement other than the blood and its animal spirits agitated by the heat of the fire which burns continually in the heart and does not differ in nature from any of the other fires which are met with in inanimate bodies.’