If it is not right, do not do it: if it is not true, do not say it.[Εἰ μὴ καθήκει, μὴ πράξῃς˙ εἰ μὴ ἀληθές ἐστι, μὴ εἴπῃς.]

Source:Meditations (ed. A. L. Burt, 1896), (tr. Long), Book 12, #17, Page 292
Find more on