§ 63. But in addition to the consideration of the various parts of nature, cosmology has commonly dealt with a radical and far-reaching alternative that appeared at the very dawn of metaphysics. Differences may arise within a world constituted of a single substance or a small group of ultimate substances, by changes in the relative position and grouping of the parts. Hence the virtue of the conception of motion. The theory which explains all differences by motions of the parts of a qualitatively simple world, is called _mechanism_. Another source of change familiar to naive experience is _will_, or the action of living creatures. According to the mechanical theory, _changes occur on account of the natural motions of the parts of matter_; according to the latter or _teleological_ conception, _changes are made by a formative agency directed to some end_. Among the early Greek philosophers, Leucippus was an exponent of mechanism. "He says that the worlds arise when many bodies are collected together into the mighty void from the surrounding space and rush together. They come into collision, and those which are of similar shape and like form become entangled, and from their entanglement the heavenly bodies arise."[161:11] Anaxagoras, on the other hand, was famed for his doctrine of the Nous, or Intelligence, to whose direction he attributed the whole process of the world. The following is translated from extant fragments of his book, "περὶ φύσεως": "And Nous had power over the whole revolution, so that it began to revolve in the beginning. And it began to revolve first from a small beginning; but the revolution now extends over a larger space, and will extend over a larger still. And all the things that are mingled together and separated off and distinguished are all known by the Nous. And Nous set in order all things that were to be and that were, and all things that are not now and that are, and this revolution in which now revolve the stars and the sun and the moon, and the air and the ether that are separated off."[162:12] [Sidenote: Dualism.]