Plato, Meno 74b

μίαν ἀρετὴν λαβεῖν κατὰ πάντων, ὥσπερ ἐν τοῖς ἄλλοις. εἰκότως γε· ἀλλʼ ἐγὼ προθυμήσομαι, ἐὰν οἷός τʼ ὦ, ἡμᾶς προβιβάσαι. μανθάνεις γάρ που ὅτι οὑτωσὶ ἔχει περὶ παντός· εἴ τίς σε ἀνέροιτο τοῦτο ὃ νυνδὴ ἐγὼ ἔλεγον, τί ἐστιν σχῆμα, ὦ Μένων; εἰ αὐτῷ εἶπες ὅτι στρογγυλότης, εἴ σοι εἶπεν ἅπερ ἐγώ, πότερον σχῆμα ἡ στρογγυλότης ἐστὶν ἢ σχῆμά τι; εἶπες δήπου ἂν ὅτι σχῆμά τι. πάνυ γε.

No, for I am not yet able, Socrates, to follow your line of search, and find a single virtue common to all, as one can in other cases. And no wonder; but I will make an effort, so far as I can, to help us onward. You understand, of course, that this principle of mine applies to everything: if someone asked you the question I put to you just now: What is figure, Meno? and you replied: Roundness; and then he said, as I did: Is roundness figure or a figure? I suppose you would answer: A figure. Certainly.