In the first half of the seventeenth century the famous philosopher Descartes, starting from Harvey's discovery of the circulation of the blood, put forward the idea that the body of man, like that of other animals, is merely an intricate machine, and that its movements take place under the same mechanical laws as the movements of an automaton of human construction. It is true that Descartes, at the same time, claimed for man the exclusive possession of a perfectly independent, immaterial soul, and held that its subjective experience, thought, was the only thing in the world of which we have direct and certain cognizance ("_Cogito, ergo sum_"). Yet this dualism did not prevent him from doing much to advance our knowledge of the mechanical life processes in detail. Borelli followed (1660) with a reduction of the movements of the animal body to purely physical laws, and Sylvius endeavored, about the same time, to give a purely chemical explanation of the phenomena of digestion and respiration; the former founded the _iatromechanical_, the latter the _iatrochemical_, school of medicine. However, these rational tendencies towards a natural, mechanical explanation of the phenomena of life did not attain to a universal acceptance and application; in the course of the eighteenth century they fell entirely away before the advance of teleological vitalism. The final disproof of the latter and a return to mechanism only became possible with the happy growth of the new science of comparative physiology in the forties of the present century.