Natural law and the law of nations

The origins of the idea of the law of nations – the ius gentium – are not to be found in the early modern period. It was first articulated by Greek and Roman classical philosophers and jurists. In the Institutes of the Roman jurist Gaius (130–180), the ius gentium is closely associated with the ius naturale . “Every people”, Gaius wrote, “that is governed by statutes and customs observes partly its own peculiar law and partly the law common to all mankind. That law which a people establishes for itself is peculiar to it, and is called ius civile as being the special law of that state, while the law that natural reason establishes among all mankind is followed by all peoples alike, and is called ius gentium as being the law observed by all mankind.” [1] Gauis’s distinction between ius naturale and ius gentium lies in the notion that the origins of this law lie in human reason while ius gentium represents its application . [2] Roman law, however, also articulated a second sense of ius gentium . [3] In the ancient world, the law applicable to persons depended on the person rather than where they lived. Athenians were governed by Athenian law, while Romans were governed by Roman law. The problem was determining what law was applicable to a person (e.g., an Athenian) living within a Roman jurisdiction, especially when it came to his relationships with people of other jurisdictions. To address potential conflicts, a Roman body of law had emerged by the first century B.C. applicable to citizen and non-citizen alike and distinguishable from the ius civile (the law specific to a particular state). Following Rome’s in the West, Isidore played a major role in preserving and codifying Gaius’ concept of the ius gentium . In his Etymologiae, Isidore listed a number of institutions (such as peace treaties and the treatment of prisoners in wartime) that he regarded as belonging to the law of nations.