 Unbeknownst to him this need could only be met effectively by the rejection of Descartes' conservation law. Spinoza nowhere calls this law explicitly into question, and his acceptance of Descartes' collision laws strongly suggests that he did in fact accept it. But he cannot have accepted the metaphysical grounds Descartes offers for it. For Descartes, substances are dependent for their existence at every moment upon God's concurrent creative activity, and since God's will is constant, he always recreates the whole of the extended world with exactly the same quantity of motion as he put there in the beginning. Each and every one of Descartes' laws of motion is for him metaphysically grounded in the immutability of the divine will. But Spinoza's God has no will, and the world is not a product of creation, in the sense in which creation follows from a decision to act in accord with a conception formed in the understanding. Spinoza's own commitment to rationalism nonetheless demands that there be some reason for the conservation of motion.