Plato, Cratylus, Theaetetus, Sophist, Statesman, Κρατύλος 89:

τί οὖν δή; ὅτι οἶμαι ἐγὼ λέγειν αὐτὸν τὸ χρυσοῦν γένοσ οὐκ ἐκ χρυσοῦ πεφυκὸσ ἀλλ’ ἀγαθόν τε καὶ καλόν. τεκμήριον δέ μοί ἐστιν ὅτι καὶ ἡμᾶσ φησιν σιδηροῦν εἶναι γένοσ. ἀληθῆ λέγεισ. οὐκοῦν καὶ τῶν νῦν οἰεί ἂν φάναι αὐτὸν εἴ τισ ἀγαθόσ ἐστιν ἐκείνου τοῦ χρυσοῦ γένουσ εἶναι;