Plato’s Theaetetus 167d:
καὶ γὰρ πολλὴ ἀλογία ἀρετῆς φάσκοντα ἐπιμελεῖσθαι μηδὲν ἀλλ᾽ ἢ ἀδικοῦντα ἐν λόγοις διατελεῖν.
“It is the height of unreasonableness that a person who professes to care for moral goodness should be consistently unjust in discussion.”
Translation by M. J. Levett
Etymologies 