As is well known, there are special tensions between Hobbesian moral psychology and Thomist moral psychology, on the one hand; and between the Hobbesian theory of free will and its Thomist rival, on the other. But what is the fundamental difference between the Thomist and Hobbesian theories of the will? This paper argues that the fundamental difference consists in three things. First, that Hobbes's theory omits, while Aquinas's theory relies on, appeals to such theoretical entities as reason, goodness, intellect, soul, and choice. Second, that Hobbes's theory admits, while Aquinas's theory denies, that brute animals have wills. Third, that Hobbes's theory denies, while Aquinas's theory grants, that the question, Is the will free? has sense. Call these differences the threefold difference, and this thesis the Threefold Difference Thesis. This thesis the paper argues for as follows. Part 2 argues that any theory of the will has nine chief features. It must answer nine questions: What is the will's nature? What is its internal structure? What is its external structure? What is its tendency? What are its functions? What is the class of will-bearers? What if anything does a volition consist in? What does voluntariness consist in? What if anything does free will consist in? Parts 3 and 4 then describe how Hobbes's and Aquinas's theories of the will answer each of these questions. Part 5 then uses these descriptions to argue for the Threefold Difference Thesis. Part 6 meets an objection to the Thesis. The objection is the thesis, inspired by the work of Thomas Pink, that the fundamental difference between the two theories is that the Thomist theory accepts a practical reason-based conception of agency, while the Hobbesian theory rejects it. Part 7 concludes the paper by considering some new questions the inquiry has broached
