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The Mechanism of Life. — So far as the life of man is like that of animals, it must be explained by purely mechanical and physical causes, particularly motion and warmth. It has been falsely supposed that the soul moves and warms the body, and is, therefore, also the physical principle of life. For every flame proves that it is not the soul which imparts motion and warmth to the body. And if the human body becomes stiff and cold after death, it suffers this change, not because it ceases to be animated by a soul, or because the soul has left it. The living body is not, as such, animated with a soul; for if so, animals would have souls, and this conflicts with the principles of Descartes' doctrine. Life does not consist in the union of soul and body; death, not in their separation. Life is not the product which produces soul, but the presupposition under which the soul enters into a union with body, the condition without which such a union cannot take place. The truth of the matter is, therefore, the direct reverse of the usual opinion. Not because the body is animated with a soul is it, therefore, alive, but, because it lives, it can be animated with a soul. The body is not stiff and cold because the soul leaves it, but the soul leaves it because it is dead. Death is the destruction of life, and is a necessary result of physical causes. Life is mechanism: death is the destruction of that mechanism, and results when the living body suffers such an injury that the whole machine stops. To the error which makes the soul the principle of life, Descartes opposes the following explanation: “Death never enters because the soul is absent, but because one of the important organs of the body is destroyed. We can, therefore, decide that the body of a living man differs from that of one who is dead, exactly as does a watch (or an automaton of any kind; i.e., a machine moved by itself) which has in itself the material principle of the motions which it is to perform, along with all the conditions necessary to its activity, and, when it is wound up, goes, – from one that is broken, in which the moving principle ceases to be active.”
The soul can be united only with a living body. Since it is of a mental nature, also, among finite beings so: far as they are knowable by us, exclusively of a human nature, this union can only take place with the body of man.
The human body, like that of animals, is a machine. Its principle of life is the fireplace in it which prepares the warmth of life, and imparts it to the whole organism, fire, whose material is the blood, and whose place is the heart. Harvey's great discovery of the motion of the blood and heart of animals explained this fundamental principle in the mechanism of life, and made an epoch in the history of biology. Descartes became acquainted with it when he was finishing his “Cosmos,” and absorbed in the investigation of the human body, and had come by his own path to a like conclusion. This doctrine appeared to him so important, and so great and evident a triumph of the mechanical physics and of the scientific method in general, that he expounded it as an example of the latter in the fifth part of his “Discours,” and, referring to Harvey's famous discovery, explained the motion of the heart and the circulation of the blood through the arteries and veins."
According to these fundamental conditions, Descartes discusses the remaining parts and function of the machine of the animal-human body. The organs of motion are the muscles, those of feeling are the nerves. The heart is the central organ of the blood and its motion, that of the nerves is the brain. Descartes represents an organ as acting between the two, whose origin and activity he characterizes as the most remarkable phenomenon of life. The finest, most mobile, most fiery particles of the blood, which are produced in the heart by a kind of distillation, ascend by mechanical laws through the arteries into the brain, and are led from thence to the nerves, and, through these, to the muscles. They cause feeling and motion in those organs, and, therefore, administer the real functions of life; and hence Descartes calls them animal spirits (esprits animaux). “The most remarkable fact in these things is the origin of animal spirits, which are like a very fine wind, or, better, a very pure and active flame, which constantly ascends in the greatest abundance from the heart into the brain, and goes thence through the nerves into the muscles, and imparts motion to all the members. But why the most mobile and the finest particles of blood, which, as such, make the best material for the animal spirits, go rather to the brain than elsewhere, is very simply explained by the fact that the arteries, which carry them to the brain, ascend from the heart in the most direct line; and if several things are striving to move at the same time in the same direction, while, as in the case of the particles of the blood, which strive to go to the brain from the left ventricle of the heart, there is not room enough for all, it follows from the laws of mechanics, which are identical with the laws of nature, that the weaker and less mobile must give place to those that are stronger, and that these alone must make their way to the brain.”
In like manner, all our involuntary motions, as in general all the activities which we have in common with animals, depend only upon the arrangement of our organs and the motion of our animal spirits, which, excited by the warmth of the heart, take their natural course into the brain, and thence into the nerves and muscles, in the same manner as the motion of a watch is produced only by the force of its spring and the form of its wheels.”
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