In this paper, I argue for the necessity of including what I call “categorical norms” in Thomas Aquinas’s account of the ground of obligation (normativity theory) by drawing on the value phenomenology of Dietrich von Hildebrand. A categorical norm is one conceptually irreducible to any non-normative concept and which obligates us irrespective of pre-existing aims, goals, or desires. I show that Thomistic normativity theory on any plausible reading of Aquinas lacks categorical norms and then raise two serious objections which constitute (...) master arguments against it. The upshot is that this theory requires reform. I end by proposing work remaining for such reform, namely, an expansion of the Thomistic metaphysic and anthropology. (shrink)