Recent studies have observed that in Grotius’ legal doctrine the intellectual ambition to create a universal rule of law (natural law) coexists with a distinctively ‘modern’ use of the vocabulary of individual rights (natural rights). In this chapter, I argue that a more careful reading of Grotius’ engagement with the Aristotelian tradition might cast new light on this traditional dichotomy, and expand our understanding of Grotius’ theory of justice. Famously, Grotius relies on the Aristotelian notion of virtue ethics to introduce the concept of aptitude, which designs a more generic account of merit and moral fitness rather than a strict, enforceable legal claim. Far from being discarded as a ‘minor’ or ‘deficient’ source of right, aptitude plays a fundamental role in this context. Through his reading and translating of the Aristotelian commentator Michael of Ephesus, I will show how Grotius’ thin conception of right as aptitude and fitness provides his natural law doctrine with a heuristic requirement for right reason.