The propagation and defense of a system of thought soon give occasion to its adherents to purify, complete, and transform it. Obscurities and contradictions are discovered, which the master has overlooked or allowed to remain, and the disciple exerts himself to remove them, while retaining the fundamental doctrines. In the system of Descartes there were two closely connected points which demanded clarification and correction, viz., his double dualism (1) between extended substance and thinking substance, (2) between created substance and the divine substance. In contrast with each other matter and mind are substances or independent beings, for the clear conception of body contains naught of consciousness, thought, representation, and that of mind nothing of extension, matter, motion. In comparison with God they are not so; apart from the creator they can neither exist nor be conceived. In every case where the attempt is made to distinguish between intrinsic and general (as here, between substance in the stricter and wider senses), an indecision betrays itself which is not permanently endured. The substantiality of the material and spiritual worlds maintained by Descartes finds an excellent counterpart in his (entirely modern) tendency to push the _concursus dei_ as far as possible into the background, to limit it to the production of the original condition of things, to give over motion, once created, to its own laws, and ideas implanted in the mind to its own independent activity; but it is hard to reconcile with it the view, popular in the Middle Ages, that the preservation of the world is a perpetual creation. In the former case the relation of God to the world is made an external relation; in the latter, an internal one. In the one the world is thought of as a clock, which once wound up runs on mechanically, in the second it is likened to a piece of music which the composer himself recites. If God preserves created things by continually recreating them they are not substances at all; if they are substances, preservation becomes an empty word, which we repeat after the theologians without giving it any real meaning.