With the appearance of Ibn Daud's masterpiece, which exhibits a more direct familiarity with the fundamental ideas of Aristotle, the method changed. The existence of God is proved directly from physics without the mediation of the doctrine of creation. Motion proves a mover, and to avoid an infinite regress we must posit an unmoved mover, that is, a first mover who is not himself moved at the same time. An unmoved mover cannot be corporeal, hence he is the spiritual being whom we call God. Ibn Daud does not make use of creation to prove the existence of God, but neither does he posit eternal motion as Aristotle does. And the result is that he has no valid proof that this unmoved mover is a pure spirit not in any way related to body. This defect was made good by Maimonides. Let us frankly adopt tentatively, he says, the Aristotelian idea of the eternity of the world, _i. e._, the eternity of matter and motion. We can then prove the existence of an unmoved mover who is pure spirit, for none but a pure spirit can have an infinite force such as is manifested in the eternal motion of the world. Creation cannot be demonstrated with scientific rigor, hence it is not safe to build so important a structure as the existence of God upon an insecure foundation. Show that eternity of the world leads to God, and you are safe no matter what the ultimate truth turns out to be concerning the origin of the world. For if the world originated in time there is no doubt that God made it. Thus Maimonides accepted provisionally the eternity of matter and motion, but provisionally only. No sooner did he prove his point, than he takes up the question of the world's origin and argues that while strict demonstration there is as yet none either for or against creation, the better reasons are on the side of creation. Gersonides, on the other hand, was a truer Aristotelian than Maimonides and he decided in favor of the eternity of matter, though not of this our world. The Jewish Mutakallimun, as we have seen, proved the existence of God from the fact that a created world implies a creator. The next step was to show that there is only one God, and that this one God is simple and not composite, and that he is incorporeal. The unity in the sense of uniqueness was shown by pointing out that dualism or pluralism is incompatible with omnipotence and perfection--attributes the possession of which by God was not considered to require proof. Maimonides, indeed, pointed out, in his opposition to the Mutakallimun, that if there is a plurality of worlds, a plurality of Gods would not necessarily be in conflict with the omnipotence and perfection of each God in his own sphere (Guide I, 75), and he inferred the unity of God from his spirituality.