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In this essay, Samuel Gregg describes the role of ius gentium, or “law of nations,” within the natural law tradition. For the Romans, the ius gentium originated in the natural law and was the class of written law that applied regardless of citizenship. The idea that the ius gentium was essentially positive law derived from the natural law persisted through the time of Aquinas. However, in the early-modern period, groups of Jesuits and Dominicans defined the ius gentium as law constituted “by the common consent of the people.” Ius gentium also came to refer to the laws which govern interactions between states, or simply to the domestic laws which were in place nearly everywhere. For Francisco Suárez, this understanding maintained the tie to the natural law because a failure to follow the ius gentium was an indicator of a corrupt society. In the modern period, Hugo Grotius and Samuel von Pufendorf treated ius gentium more as a law between states (thus weakening its connection to the natural law), though both viewed it to be higher than mere convention. By the 18th century, the concept of ius gentium as the common law of humanity was largely marginalized. Only faint echoes of the early-modern natural law tradition remained as the conception of the law of nations became increasingly focused on the application of the laws in Hobbes’s “state of nature” to state relations, and in particular to international commercial relations.