This article concerns Aquinas’ practical doctrine on two philosophical difficulties underlying much contemporary ethical debate. One is Hume’s Is-ought thesis and the other is its radical consequence, Moore’s Open-question argument. These ethical paradoxes appear to have their roots in epistemological scepticism and in a deficient anthropology. A possible response to them can be found in that a) Aquinas defends the substantial unity and rationality of the human being; b) Thomistic natural law is a natural consequence of the rational being; c) Thomistic human intellect is essentially theoretical and practical at the same time; d) Aquinas’ human reason naturally performs three main operations. First, to apprehend the intellecta and universal notions ens, verum and bonum. Second, to formulate the first theoretical and practical principles. Third, to order that the intellectum and universal good be done and the opposite avoided. For these reasons, Thomistic philosophical response to both predicaments will not be exclusively ethical, but will embrace ontology, anthropology and epistemology. Aquinas’ moral philosophy is fundamentally different from ethics that qualifies actions as good either by mere social consensus (contractualism) or just by calculating its consequences (consequentialism).