From the early reception of Thomas Aquinas up to the present, many have interpreted his theory of liberum arbitrium to imply intellectual determinism: we do not control our choices, because we do not control the practical judgments that cause our choices. In this paper we argue instead that he rejects determinism in general and intellectual determinism in particular, which would effectively destroy liberum arbitrium as he conceives of it. We clarify that for Aquinas moral responsibility presupposes liberum arbitrium and thus (...) the ability to do otherwise, although the ability to do otherwise applies differently to praise and blame. His argument against intellectual determinism is not straightforward, but we construct it by analogy to his arguments against other deterministic threats. The non-determinism of the intellect’s causality with respect to the will results from his claims that practical reasoning is defeasible and that the reasons for actions are not contrastive reasons. (shrink)