 Aquinas discusses promises indirectly, in his treatment of vows, which he takes are promises made to God. His adaptation of Aristotelian virtue theory to the precepts of the natural law made for some important changes in the theory—principally a shift in focus to the will of the promisor as the agent of obligation. For Aquinas, a promise is essentially the agent “binding” himself (i.e., generating an obligation) by act of will: