Page:A Brief History of Modern Philosophy.djvu/55

52 be mechanics. The organism must be subject to the general law of matter. Harvey’s discovery of the circulation of the blood (1628) strengthened Descartes’ conviction. Descartes did much to suppress the fruitless theory of vitalism which explained organic phenomena by the assumption of a specific vital energy. In the department of nerve physiology, like Harvey in the doctrine of the circulation of the blood, he is a pioneer because he was the first to describe what is now called reflex action, i.e. muscular activity resulting directly from an objective stimulus without the intervention of any attendant consciousness. Descartes ascribed consciousness to man alone; he regarded animals as mere machines.

The human soul interacts with the brain, or, to be more exact, with a distinct part of the brain (the pineal gland, glandula pinealis), which, in Descartes’ opinion, was centrally located, and it does not consist of pairs, like the other parts of the brain. The “vital spirits” (the delicate fluid, which, according to the physiology of the age, inherited from antiquity, pours through the nerves) strike this pineal gland and the impact translates it to the soul, thus giving rise to sensations. If the soul on the other hand strikes the pineal gland it can produce changes in the tendencies of the “vital spirits” and thus give rise to muscular activity.—Here Descartes contradicts his own doctrine of the persistence of motion; for if the pineal gland strikes the soul, a loss of motion must result, and, conversely, if the soul excites motion in the pineal gland, new motion must arise. He of course limits the action of the soul to the mere matter of producing a change of tendency; but this requires him to postulate an arbitrary exception to the principle of inertia.