Historians commonly date the beginning of early modern epistemology and metaphysics from Descartes’s attempt in the Meditations to find a foundation for knowledge that is immune to skeptical challenge for an individual self-critical mind. There is no comparable consensus about when early modern ethical philosophy begins, but, as J. B. Schneewind has argued, it makes sense to link it similarly to an engagement with forms of ethical skepticism in the writings of Montaigne in the late sixteenth century and Hugo Grotius in the early seventeenth. If one were to seek a parallel canonical moment, one might do no better than a passage in Grotius’s On the Law of War and Peace (1625), in which Grotius puts into the mouth of the ancient skeptic Carneades the challenge that “[T]here is no law of nature, because all creatures … are impelled by nature towards ends advantageous to themselves … [C]onsequently, there is no justice, or if such there be, it is supreme folly, since one does violence to his own interests if he consults the advantage of others.”

To appreciate the force of this challenge, we must know what Grotius and his contemporaries would have understood by a “law of nature.” Natural laws (of the normative or ethical sort) were thought of as universal norms that impose obligations on anyone who is capable of following them, on all moral agents, rather than on citizens of a more specific jurisdiction. And, differently from positive law, they were thought to require no positing, legislative act, at least no human one.