Ius in Justinian's Digest
Justinian, Digest 22.214.171.124 (533 A.D.) Ulpian on the First Book of the Institutes (ca. 230 A.D.): Iuri operam daturum prius nosse oportet, unde nomen Iuris descendat. Est autem a Iustitia appellatum: nam, ut eleganter Celsus definit, Ius est ars boni et aequi. Cuius merito quis nos sacerdotes appellet: iustitiam namque colimus et boni et aequi notitiam profitemur, aequum ab iniquo separantes, licitum ab illicito discernentes. (A law student at the beginning of his studies should know from where "ius" comes. Its name is taken from justice. For, in the elegant words of Celsus, "ius" is the art of the good and the equitable. Consequently we jurists are appropriately called priests because we cultivate justice, and we seek knowledge of the good and the equitable. We mark the difference between equitable and inequitable and determine what is licit and not licit).
Dig.1.1.9 Gaius 1 inst. 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 Ius that is common to all men. The Ius which each people have established for itself in its own city is called Ius civile, which is the Ius of each city. That Ius which naturalis ratio establishes for all men and which all men observe is called Ius gentium, as it is the Ius that all men observe).
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).
Dig.126.96.36.199 Ulpianus 1 reg. Iuris praecepta sunt haec: honeste vivere, alterum non laedere, suum cuique tribuere (The commands of Ius are these: to live honestly, to not injure <the rights> of others, and to give to each person his <Ius>).
Dig.188.8.131.52 Ulpianus 1 reg. Iuris prudentia est divinarum atque humanarum rerum notitia, iusti atque iniusti scientia. (Jurisprudence is the understanding of divine and human affairs, and the the skill of recognizing justice and injustice).
Dig.1.1.11 Paulus 14 ad sab. Ius pluribus modis dicitur: uno modo, cum id quod semper aequum ac bonum est Ius dicitur, ut est Ius naturale. altero modo, quod omnibus aut pluribus in quaque civitate utile est, ut est Ius civile . . . Alia significatione Ius dicitur locus in quo Ius redditur, appellatione collata ab eo quod fit in eo ubi fit. Quem locum determinare hoc modo possumus: ubicumque praetor salva maiestate imperii sui salvoque more maiorum ius dicere constituit, is locus recte ius appellatur (The term "ius" can be used in several ways. In one way "ius" means what is always equitable and good, as "Ius naturale". In another way what is in the interest of all or of many in a state (civitas), such as the "Ius civile" . . . Yet another meaning of "ius" is to describe the place in which "ius" is vindicated, the name having been given by him who renders "ius" on the place where he does it. We can know where that place is by wherever the praetor decides to exercise his jurisdiction, preserving the majesty of his authority and respecting the "mos" of our ancestors. That place is correctly called "ius.") CL Introduction