Descartes famously espouses modal voluntarism, the doctrine that God freely creates the eternal truths. God has chosen to make it true that two plus two equals four, for instance, but he could have chosen otherwise. Why, though, does Descartes endorse modal voluntarism? Many commentators have noted that he regularly appeals to divine omnipotence to justify his doctrine. This strategy is usually thought to be unsuccessful, however, because it seems to presuppose—question-beggingly—that the eternal truths are in the scope of God’s power. (...) This paper argues that Descartes’s appeal to divine omnipotence has more going for it than meets the eye. Like many other medieval and early modern philosophers, Descartes assumes that God has the power to control everything that remains unfixed by his own essence. At the same time, though, he denies that the eternal truths are fixed by, or grounded in, the divine essence. The combination of these two commitments leads to modal voluntarism. The paper also argues that the Principle of Sufficient Reason (PSR) may play an important role in the reasoning that leads Descartes to endorse modal voluntarism. This is surprising, given that the PSR appears to be deeply at odds with modal voluntarism. (shrink)