Zofingen, Stadtbibliothek, Pa 32, fol. 88v  (9th Century)

Isidore of Seville (†636), Etymologies Book 5 - Gratian, Decretum D.1 c.7

Gratian was the first canonist to insert a large number of canons from the Etymologies Book 5 (De legibus) into his canonical collection ca. 1135. 

Pennington, ALex naturalis and Ius naturale,@ Crossing Boundaries at Medieval Universities, ed. S. E. Young (Education and Society in the Middle Ages and Renaissance, 36;  Leiden-Boston: Brill, 2011) 227-253

Natural ius is common to all peoples.  It has its origins in the instinct of nature, not in any constitution as in the union of men and women, the procreation and raising of children, the common possessions of all persons, the equal liberty of all persons, the acquisition of things that are taken from the heavens, earth, or sea, or the return of property or money that has been deposited or entrusted.  This also includes the right to repel violence with force. These and similar things are never unjust but are natural and equitable