Omnes populi, qui legibus et moribus reguntur, partim suo proprio, partim communi omnium hominum iure utuntur. Nam quod quisque populus ipse sibi ius constituit, id ipsius proprium civitatis est vocaturque ius civile, quasi ius proprium ipsius civitatis: quod vero naturalis ratio inter omnes homines constituit, id apud omnes peraeque custoditur vocaturque ius gentium, quasi quo iure omnes gentes utuntur

(All peoples who are ruled by laws and long-standing custom partly use their own laws and partly the law that is common to all men.  The ius that each nation has constituted for itself  for each city is called the ius civile; almost as if  it were a ius proprium of that city.  What, however, the natural reason of men establish and is used by all men equally, is called the ius gentium, almost as if all human beings use that ius).

Dig.1.1.10pr. Ulpianus 1 reg. Iustitia est constans et perpetua voluntas ius suum cuique tribuendi (Justice is the constant and perpetual will of giving everyone their Ius).