The human will can, by common agreement, make a thing to be just provided it be not, of itself, contrary to natural justice, and it is in such matters that positive right has its place.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic justice agreement
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/18755/pg18755-images.html

Context

“but since it happens sometimes that man's will is unrighteous there are cases in which a deposit should not be restored, lest a man of unrighteous will make evil use of the thing deposited: as when a madman or an enemy of the common weal demands the return of his weapons. Reply Obj. 2: The human will can, by common agreement, make a thing to be just provided it be not, of itself, contrary to natural justice, and it is in such matters that positive right has its place. Hence the Philosopher says (Ethic. v, 7) that "in the case of the legal just, it does not matter in the first instance whether it takes one form or another, it only matters when once it is laid down."” source