[5] The jurist Gaius defined the ius gentium as what "natural reason has established among all peoples":[6] Every people (populus) that is governed by statutes and customs (leges et mores) observes partly its own peculiar law and partly the common law of all mankind.
[8] Cicero[9] distinguished between things that are written and those that are unwritten but upheld by the ius gentium or the mos maiorum, "ancestral custom".
[11] A person driven into exile, for instance, lost his legal standing as a Roman citizen, but was supposed to retain the basic protections extended to all human beings under the ius gentium.
[13] Slavery, for instance, was supported by the ius gentium, even though under natural law all are born free (liberi).
[15] Hermogenianus, a Roman jurist of the second half of the 3rd century, described the ius gentium as comprising wars, national interests, kingship and sovereignty, rights of ownership, property boundaries, settlements, and commerce, "including contracts of buying and selling and letting and hiring, except for certain contractual elements distinguished through ius civile".
"[26] A key passage pertaining to what Romans understood as "international law" is presented by Livy, as spoken by an envoy of King Antiochus:[27] There were three kinds of treaties (foedera, singular foedus), he said, by which states and kings concluded friendships (amicitiae): one, when in time of war terms (leges) were imposed upon the conquered; for when everything was surrendered to him who was the more powerful in arms, it is the victor's right and privilege to decide what of the conquered's property he wishes to confiscate; the second, when states that are equally matched in war conclude peace and friendship on terms of equality; under these conditions demands for restitution are made and granted by mutual agreement, and if the ownership of any property has been rendered uncertain by the war, these questions are settled according to the rules of traditional law or the convenience of each party; the third exists when states that have never been at war come together to pledge mutual friendship in a treaty of alliance; neither party gives or accepts conditions; for that happens when a conquering and a conquered party meet.