Laudator Temporis Acti

Plutarch, How to Profit by One's Enemies 4 = Moralia 88 D (tr. Frank Cole Babbitt):

Enter within the portals of your own soul, look about to see if there be any rottenness there, lest some vice lurking somewhere within whisper to you the words of the tragedian:

Wouldst thou heal others, full of sores thyself?

If you call your enemy uneducated, strive to intensify in yourself the love of learning and industry; if you call him a coward, rouse even more your self-reliance and manliness; if you call him unchaste and licentious, obliterate from your soul whatever trace of devotion to pleasure may be lurking there unperceived. For there is nothing more disgraceful or painful than evil-speaking that recoils upon its author.

ἐνδύου τῇ ψυχῇ, περισκόπει τὰ σαθρά, μή τίς σοί ποθεν ὑποφθέγγηται κακία τὸ τοῦ τραγῳδοῦ

ἄλλων ἰατρὸς αὐτὸς ἕλκεσιν βρύων.

ἂν ἀπαίδευτον εἴπῃς, ἐπίτεινε τὸ φιλομαθὲς ἐν σεαυτῷ καὶ φιλόπονον· ἂν δειλόν, ἔγειρε μᾶλλον τὸ θαρραλέον καὶ ἀνδρῶδες· κἂν ἀσελγῆ καὶ ἀκόλαστον, ἐξάλειφε τῆς ψυχῆς εἴ τι λανθάνον ἐστὶ φιληδονίας ἴχνος. οὐδὲν γὰρ αἴσχιόν ἐστι βλασφημίας παλινδρομούσης οὐδὲ λυπηρότερον.