This book places Descartes metaphysics into a broader historical-philosophical context to reveal not only why, but also how Descartes opens the possibility for the necessity of designating God as causa sui. Descartes takes over the term from the Scholastics, and the present author underscores the significance of, first, examining the term as it was understood in Scholastic philosophy, and then seeks to follow, from Thomas Aquinas to Descartes, the radical change in thinking the metaphysical problem that is the subject of this far-reaching study.