Plato, Meno 75a

πειρῶ λέγειν. εἰ οὖν τῷ ἐρωτῶντι οὕτως ἢ περὶ σχήματος ἢ χρώματος εἶπες ὅτι ἀλλʼ οὐδὲ μανθάνω ἔγωγε ὅτι βούλει, ὦ ἄνθρωπε, οὐδὲ οἶδα ὅτι λέγεις, ἴσως ἂν ἐθαύμασε καὶ εἶπεν· οὐ μανθάνεις ὅτι ζητῶ τὸ ἐπὶ πᾶσιν τούτοις ταὐτόν; ἢ οὐδὲ ἐπὶ τούτοις, ὦ Μένων, ἔχοις ἂν εἰπεῖν, εἴ τίς σε ἐρωτῴη· τί ἐστιν ἐπὶ τῷ στρογγύλῳ καὶ εὐθεῖ καὶ ἐπὶ τοῖς ἄλλοις, ἃ δὴ σχήματα καλεῖς, ταὐτὸν ἐπὶ πᾶσιν; πειρῶ εἰπεῖν, ἵνα καὶ γένηταί σοι μελέτη πρὸς τὴν περὶ τῆς ἀρετῆς ἀπόκρισιν.

either about figure or about color, you had replied: Why, I don’t so much as understand what you want, sir, or even know what you are saying: he might well have shown surprise, and said: Do you not understand that I am looking for that which is the same common element in all these things? Or would you still be unable to reply, Meno, if you were approached on other terms, and were asked: What is it that is common to the round and the straight and everything else that you call figures—the same in all? Try and tell me it will be good practice for your answer about virtue.