Plato, Meno 84d

φύλαττε δὲ ἄν που εὕρῃς με διδάσκοντα καὶ διεξιόντα αὐτῷ, ἀλλὰ μὴ τὰς τούτου δόξας ἀνερωτῶντα.λέγε γάρ μοι σύ· οὐ τὸ μὲν τετράπουν τοῦτο ἡμῖν ἐστι χωρίον; μανθάνεις; ἔγωγε. ἕτερον δὲ αὐτῷ προσθεῖμεν ἂν τουτὶ ἴσον; ναί. καὶ τρίτον τόδε ἴσον ἑκατέρῳ τούτων; ναί. οὐκοῦν προσαναπληρωσαίμεθʼ ἂν τὸ ἐν τῇ γωνίᾳ τόδε; πάνυ γε. ἄλλο τι οὖν γένοιτʼ ἂν τέτταρα ἴσα χωρία

and do not teach him; and be on the watch to see if at any point you find me teaching him or expounding to him, instead of questioning him on his opinions. Tell me, boy: here we have a square of four feet, have we not? You understand? Yes. And here we add another square equal to it? Yes. And here a third, equal to either of them? Yes. Now shall we fill up this vacant space in the corner? By all means. So here we must have four equal spaces?