Perfectionism is the view that what is intrinsically good is the fulfillment of human nature or the development and exercise of the characteristic human capacities. An important objection to the theory is what Gwen Bradford calls the “Deep Problem”: explaining why nature-fulfillment is good. We argue that situating perfectionism within a Thomistic metaethical framework and adopting Aquinas's account of the metaphysical “convertibility” of being and goodness gives us a solution to the Deep Problem. In short, the fulfillment of human nature (...) consists in the actualization of human potentialities or fullness of human being, and because being is ultimately the same thing as goodness, the fulfillment of human nature is good. We show that Thomistic perfectionism meets the requirements for an answer to the Deep Problem, provides the best explanation possible for the goodness of nature fulfillment, and is a natural foundation for perfectionist theories of value. (shrink)