Gratian's Decretum (ca. 1120-1140)

Hierarchy of Laws

1. Divine Law

2. Natural Law - Ius gentium

3.  Custom (Mos, consuetudo)

The Human Race is ruled by two things:  namely, natural law (ius naturale) and ancient usages of peoples (mos).  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