1. Let us consider the first hypothesis. It is claimed that gravitation, or universal attraction, is the great conserving and sustaining principle of the universe. A stone falls to the earth, a round body rolls along a plane inclined toward the horizon; a liquid mass, as a brook or a large river, flows on the sloping surface which forms its bed. All these phenomena are the varied manifestation of a universal _tendency_ in all bodies to fall one toward the other. In virtue of this tendency the great orbs which hang suspended in space gravitate toward one another; the moon and the earth fall toward each other, and they both gravitate toward the sun. All the planets of our solar system continually act one on the other, and on the immense sphere which shines at their common focus. By its enormous mass, the sun keeps all of them in their orbits. If we ask why one body _falls_ toward another which is more than ninety millions of miles off, in preference to moving in any other direction, the answer given is that, "Every particle of matter in the universe _attracts_ every other particle with a force whose direction is that of the line joining the two, and whose magnitude is directly as the product of their masses, and inversely as the square of their distance from each other." This force of attraction is the universal bond which holds the universe together, and sustains its physical life. To the superficial thinker, the language of the Newtonian philosophy appears to sanction the materialistic notion that gravitation and attraction are active powers essential to and inherent in matter. Such, however, was by no means the doctrine of Newton, and he was careful to guard his readers against any such misapprehension of his meaning. "The words attraction, repulsion, or tendencies of whatever kind toward a centre, I use indifferently and without distinction for each other, considering these forces not physically but metaphysically. Wherefore let not the reader suppose that by words of this kind I any where mean a species or mode of action, or cause, or physical reason; or that I really and in a physical sense assign forces to centres (which are only mathematical points), even though I may say that centres _attract_, or that _forces belong to centres_."[271]