THE SUBJUGATION OF PHYSIOLOGY.--It was an Englishman who provided the necessary facts to fill the gravest gap in the mechanical theory. It was already known in the previous century that the blood of animals circulated throughout the body; the existence and use of veins and heart-valves was also known, but it was William Harvey (1578-1657) who discovered the heart to be the organ responsible for _maintaining_ the circulation of the blood, by purely mechanical means. This was a fact of the utmost significance. In the sphere of physiology, where theories about mysterious powers of blood or soul had been hitherto authoritative, it effected a revolution. Indeed it is true to say that Harvey "is to physiology what Galileo was to physics." He proved that "the general laws of motion are valid within as well as without the organism"--an important extension of the mechanical theory. DESCARTES.--Among the leading men who accepted Harvey's theory, one of the first was Rene Descartes (1596-1650). Well might this thinker welcome it, for it was a most important contribution to the imposing philosophic fabric for which he was industriously collecting materials. Descartes, apart from his philosophical speculations, is an interesting character, being a Frenchman of noble birth who was educated by the Jesuits, saw something of contemporary life in Paris, served as a military officer in Holland and Germany, and made some original discoveries in mathematics. The mathematical mind, accustomed as it is to deal with highly abstract ideas, takes kindly to metaphysics. And it very often solves the mystery of the universe by expressing all its contents in mathematical terms. Such, at least, was Descartes' method. The simplest and clearest ideas which we can have of anything are mathematical, i.e. extension and mobility. And it is by concentrating our attention upon this simple and mathematical aspect of things that we shall arrive at a proper understanding of all that goes on in the material world. UNIVERSALITY OF MATHEMATICS.--A phenomenon was, in Descartes' eyes, "explained" only when a "cause" which is its exact _mathematical equivalent_, has been indicated. The "cause" and the "effect" are two sides of a mathematical equation (_Causa aequat effectum_). Anything that happens in the material world (the fall of a stone, the beat of a heart, the rising of the sun) is really nothing more than a redistribution of portions of that sum of motion which, once generated at the Creation, has remained unaltered, and unalterable, in the universe ever since. The sum of motion is constant, there can be no addition to or subtraction from it. In this sense it would be true that "there is nothing new under the sun": only ever-new distributions of the old.