Grotius in the Prolegomena examines the reasons for the validity of natural law in a polemical discussion with the sceptical critic of Stoical philosophy, Carneades. The latter, in one of his speeches in Rome, asserted that all human conduct is motivated by one’s own utility and that justice does not exist. Grotius rejects these sceptical objections because people have an innate desire for community and, on the basis of their reason, are able to conduct themselves according to general rules. These rules are, as a series of norms of natural right, evident in themselves and unalterable. They would even be valid were there no God. Natural law is, according to Grotius, superordinate to the civil law and is the source of international law.
The Prolegomena opens with the remark that we lack a systematic work dealing with international law (ius gentium; § 1), although classical texts provide numerous examples of how ancient Romans and Greeks understood the question of a justlywaged war (§ 2–27). By providing a solution to this question Grotius wishes to help his times (§ 28–32): some of his contemporaries, after all, disparaged all use of weapons. After presenting the themes of each of the three books (§ 33–35) Grotius explained how he classifi ed his sources according to their usefulness in drawing up a groundplan of the natural elements of international law (§ 36–55) and he characterised the principles of the work (§ 56–61).