Köln, Erzbischöfliche Diözesan- und Dombibliothek 127, fol. 9r
Link to the Manuscripts in the Cologne Cathedral Chapter Library
Gratian, De legibus, Distinctiones 1-20
The Human Race is ruled by two things: namely, natural law (ius naturale) and ancient usages of peoples (mos). (Humanum genus duobus regitur, naturali videlicet iure et moribus) Natural law is what is contained in the Law (Lex) and the Gospel. By it, each person is commanded to do to others what he wants done to himself and is prohibited from inflicting on others what he does not want done to himself. Thus Christ said in the Gospel: "Whatever you want men to do to you, do so to them. This is the Law and the Prophets." [Matthew 7:12, cf. Luke 6:31].
Roman Law Source of Gratian's Opening Dictum
Dig.1.1.9 Gaius 1 inst. Omnes populi, qui legibus et moribus reguntur, partim suo proprio, partim communi omnium hominum iure 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)