My aim in this article is to analyze the concept of causa sui as Descartes presents it in his replies to Caterus’s and Arnauld’s objections. In the wake of this analysis I will show that there is a necessary connection between the Cartesian theses of the free creation of eternal truths and that of God as self-caused. As a consequence it will be clear that Descartes introduces the thesis of God as causa sui not just to oppose a tradition before him, but rather to respond to a need of his own system: God as causa sui is the only way available to him to guarantee his doctrine of the free creation of the eternal truths.