Descartes saw that the discoveries of Galileo meant that the remotest parts of the universe were governed by mechanical laws; while those of Harvey meant that the same laws presided over the operations of that portion of the world which is nearest to us, namely, our own bodily frame.  And crossing the interval between the centre and its vast circumference by one of the great strides of genius, Descartes sought to resolve all the phaenomena of the universe into matter and motion, or forces operating according to law.[71] This grand conception, which is sketched in the “Discours,” and more fully developed in the “Principes” and in the “Traite de l’Homme,” he worked out with extraordinary power and knowledge; and with the effect of arriving, in the last-named essay, at that purely mechanical view of vital phaenomena towards which modern physiology is striving.

Let us try to understand how Descartes got into this path, and why it led him where it did.  The mechanism of the circulation of the blood had evidently taken a great hold of his mind, as he describes it several times, at much length.  After giving a full account of it in the “Discourse,” and erroneously describing the motion of the blood, not to the contraction of the walls of the heart, but to the heat which he supposes to be generated there, he adds:—­

“This motion, which I have just explained, is as much the necessary result of the structure of the parts which one can see in the heart, and of the heat which one may feel there with one’s fingers, and of the nature of the blood, which may be experimentally ascertained; as is that of a clock of the force, the situation, and the figure, of its weight and of its wheels.”

But if this apparently vital operation were explicable as a simple mechanism, might not other vital operations be reducible to the same category?  Descartes replies without hesitation in the affirmative.

“The animal spirits,” says he, “resemble a very subtle fluid, or a very pure and vivid flame, and are continually generated in the heart, and ascend to the brain as to a sort of reservoir.  Hence they pass into the nerves and are distributed to the muscles, causing contraction, or relaxation, according to their quantity.”

Thus, according to Descartes, the animal body is an automaton, which is competent to perform all the animal functions in exactly the same way as a clock or any other piece of mechanism.  As he puts the case himself:—­

“In proportion as these spirits [the animal spirits] enter the cavities of the brain, they pass thence into the pores of its substance, and from these pores into the nerves; where, according as they enter, or even only tend to enter, more or less, into one than into another, they have the power of altering the figure of the muscles into which the nerves are inserted, and by this means of causing all the limbs to move.  Thus, as you may have seen in the grottoes and the fountains