For thirty years now there has been considerable debate concerning the foundations of modern natural law theory, with Richard Tuck emphasising the role self-preservation plays in anchoring Grotius's system and his critics pointing to the contribution of a principle of sociability. With reference to recent contributions in the literature on Stoicism from Julia Annas, A. A. Long and Tad Brennan, I argue that Grotius's use of the outline of Stoic ethics from Book III of Cicero's De finibus is crucial for understanding the nature of his argument. Drawing on Cicero's presentation of the Stoics' oikeiosis (Latin: appetitus societatis) helps Grotius to generate an argument which issues not in any demand for altruism, charity or mutual aid, but rather for organising justice around very strong protections for private property. The argument remains one about human sociability, however, and ought not to be mistaken for an account of self-interest, nor for a doctrine with substantially Epicurean roots.