Gratian's Decretum (ca. 1120-1140)
Hierarchy of Laws
1. Divine Law
2. Natural Law - Ius gentium
3. Custom (Mos, consuetudo)
Human Race is ruled by two things: namely, natural law
and ancient usages of peoples
Natural law is what is contained in the Law
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