Abstract: It is widely believed that Hugo Grotius played a central role in shaping a selfconsciously “modern” form of ethical philosophy in the seventeenth century. There is disagreement, however, about what is distinctively new or “modern” in Grotius’s thought. Moreover, Terence Irwin has argued recently that Grotius’s ideas marked no significant departure from an earlier “Aristotelian naturalist” orthodoxy. This essay argues that there is indeed something importantly new in Grotius: a theory of perfect rights as legitimate demands, which Grotius places at the heart of a complementary conception of morality. Morality, as Grotius conceives it, consists of “obligations” whose binding force cannot be reduced to reasons that recommend or “counsel” conduct, however strongly. Thus arose the modern problem: how to account for morality’s distinctive normative force or “authority”.