Man is not bound to do gratuitously whatever he can do from motives of mercy: else no man could lawfully sell anything, since anything may be given from motives of mercy.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic mercy motive
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

“cliii ad Macedon.) that "it is customary to demand from them restitution of what they have extorted by a wicked excess, but not what has been given to them in accordance with a commendable custom." Reply Obj. 1: Man is not bound to do gratuitously whatever he can do from motives of mercy: else no man could lawfully sell anything, since anything may be given from motives of mercy. But when a man does give a thing out of mercy, he should seek, not a human, but a Divine reward. In like manner an advocate, when he mercifully pleads the cause of a poor man, should have in view not a human but a Divine meed;” source