Plato, Meno 74d

ἂν ἄλλα, ἃ οὐδὲν ἧττον τυγχάνει ὄντα χρώματα τοῦ λευκοῦ; ναί. εἰ οὖν ὥσπερ ἐγὼ μετῄει τὸν λόγον, καὶ ἔλεγεν ὅτι ἀεὶ εἰς πολλὰ ἀφικνούμεθα, ἀλλὰ μή μοι οὕτως, ἀλλʼ ἐπειδὴ τὰ πολλὰ ταῦτα ἑνί τινι προσαγορεύεις ὀνόματι, καὶ φῂς οὐδὲν αὐτῶν ὅτι οὐ σχῆμα εἶναι, καὶ ταῦτα καὶ ἐναντία ὄντα ἀλλήλοις, ὅτι ἐστὶν τοῦτο ὃ οὐδὲν ἧττον κατέχει τὸ στρογγύλον ἢ τὸ εὐθύ, ὃ δὴ ὀνομάζεις σχῆμα

you would tell him of others that are colors just as much as white? Yes. Now suppose that, like me, he pursued the argument and said: We are always arriving at a variety of things, but let me have no more of that: since you call these many things by one single name, and say they are figures, every one of them, even when they are opposed to one another, tell me what is that which comprises round and straight alike, and which you call figure—