Justinian, Institutes 1.1 (533 A.D.)

Iustitia est constans et perpetua voluntas ius suum cuique tribuens.  Iuris prudentia est divinarum atque humanarum rerum notitia, iusti atque iniusti scientia.   

Justice is the constant and perpetual will to render to every man his due. Jurisprudence is the knowledge of divine and human affairs and knowledge of what is just and what is unjust. (Matthew 7:12 and Luke 6:31).

Ambrogio Lorenzetti, Bad Government, Palazzo Pubblico, Siena,  ca. 1340 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.  It is derived from justice.  For, in the elegant words of Celsus (ca. 100 A.D.), "ius" is the art of the good and the equitable.  Consequently we jurists are 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.