Plato, Meno 73d

εἴπερ ἕν γέ τι ζητεῖς κατὰ πάντων. ἀλλὰ μὴν ζητῶ γε. ἀλλʼ ἆρα καὶ παιδὸς ἡ αὐτὴ ἀρετή, ὦ Μένων, καὶ δούλου, ἄρχειν οἵω τε εἶναι τοῦ δεσπότου, καὶ δοκεῖ σοι ἔτι ἂν δοῦλος εἶναι ὁ ἄρχων; οὐ πάνυ μοι δοκεῖ, ὦ Σώκρατες. οὐ γὰρ εἰκός, ὦ ἄριστε· ἔτι γὰρ καὶ τόδε σκόπει. ἄρχειν φῂς οἷόν τʼ εἶναι. οὐ προσθήσομεν αὐτόσε τὸ δικαίως, ἀδίκως δὲ μή; οἶμαι ἔγωγε· ἡ γὰρ δικαιοσύνη, ὦ Σώκρατες, ἀρετή ἐστιν.

if you want some single description to cover all cases. That is just what I am after. But is virtue the same in a child, Meno, and in a slave—an ability to govern each his master? And do you think he who governed would still be a slave? I should say certainly not, Socrates. No, indeed, it would be unlikely, my excellent friend. And again, consider this further point: you say it is to be able to govern; shall we not add to that—justly, not unjustly? Yes, I think so; for justice, Socrates, is virtue.