Calumniae non temere credundum
Lucian of Samosata
Lucian, Vol. 1. Harmon, A. M., editor. London: William Heinemann, Ltd.; Cambridge, MA: Harvard University Press, 1913.
Therefore, when a man comes and tells you a thing of this sort, you must investigate the matter on its own merits, without regarding the years of the speaker or his standing, or his carefulness in what he says; for the more plausible a man is, the closer your investigation should be. You should not, then, put faith in another's judgment, or rather (as you would be doing), in the accuser’s want of judgment, [*](Literally, "in the accuser’s hatred.” To secure something like the word-play in the Greek, the sense had to suffer slightly.) but should reserve to yourself the province of investigating the truth, accrediting the slanderer with his envy and conducting an open examination into the sentiments of both men; and you should only hate or love a man after you have put him to the proof. To do so before that time, influenced by the first breath of slander—Heavens! how