Plato, Meno 72c

εἰ οὖν εἶπον μετὰ ταῦτα· τοῦτο τοίνυν μοι αὐτὸ εἰπέ, ὦ Μένων· ᾧ οὐδὲν διαφέρουσιν ἀλλὰ ταὐτόν εἰσιν ἅπασαι, τί τοῦτο φῂς εἶναι; εἶχες δήπου ἄν τί μοι εἰπεῖν; ἔγωγε. οὕτω δὴ καὶ περὶ τῶν ἀρετῶν· κἂν εἰ πολλαὶ καὶ παντοδαπαί εἰσιν, ἕν γέ τι εἶδος ταὐτὸν ἅπασαι ἔχουσιν διʼ ὃ εἰσὶν ἀρεταί, εἰς ὃ καλῶς που ἔχει ἀποβλέψαντα τὸν ἀποκρινόμενον τῷ ἐρωτήσαντι ἐκεῖνο δηλῶσαι, ὃ τυγχάνει

And if I went on to say: Well now, there is this that I want you to tell me, Meno: what do you call the quality by which they do not differ, but are all alike? You could find me an answer, I presume? I could. And likewise also with the virtues, however many and various they may be, they all have one common character whereby they are virtues, and on which one would of course be wise to keep an eye when one is giving a definitive answer to the question of what virtue really is.