Jus gentium

The law of nations. That law which natural reason has established among all men is equally observed among all nations, and is called the "law of nations," as being the law which all nations use. Inst. 1, 2, 1; Dig. 1, 1, 9; 1 Bl. Comm. 43; 1 Kent, Comm. 7; Mackeld. Rom. Law, § 125. Although this phrase had a meaning in the Roman law which may be rendered by our expression "law of nations," it must not be understood as equivalent to what we now call "international law," its scope being much wider. It was originally a system of law, or more properly equity, gathered by the early Roman lawyers and magistrates from the common ingredients in the customs of the old Italian tribes,—those being the nations, gentes, whom they had opportunities of observing,—to be used in cases where the jus civile did not apply; that is, in cases between foreigners or between a Roman citizen and a foreigner. The principle upon which they proceeded was that any rule of law which was common to all the nations they knew of must be intrinsically consonant to right reason, and therefore fundamentally valid and just. From this it was an easy transition to the converse principle, viz., that any rule which instinctively commended itself to their sense of justice and reason must be a part of the jus gentium. And so the latter term came eventually to be about synonymous with "equity," (as the Romans understood it,) or the system of praetorian law. Modern jurists frequently employ the term "jus gentium privatum" to denote private international law, or that subject which is otherwise styled the "conflict of laws;" and "jus gentium publicum" for public international law, or the system of rules governing the intercourse of nations with each other as persons.

the law of nations, (q. Vv.)