There is no duty more indispensible than that of returning a kindness.
Marcus Tullius Cicero, Cyrus R. Edmonds (1863). “Three Books of Offices; Or, Moral Duties: Also His Cato Major, an Essay on Old Age; Laelius, an Essay on Friendship; Paradoxes; Scipio's Dream; and Letter to Quintus on the Duties of a Magistrate. Literally Translated, with Notes, Designed to Exhibit a Comparative View of the Opinions of Cicero, and Those of Modern Moralists and Ethical Philosophers”, p.27