Plato, Meno 82d

ναί. ἐπειδὴ δὲ δυοῖν ποδοῖν καὶ ταύτῃ, ἄλλο τι ἢ δὶς δυοῖν γίγνεται; γίγνεται. δυοῖν ἄρα δὶς γίγνεται ποδῶν; ναί. πόσοι οὖν εἰσιν οἱ δύο δὶς πόδες; λογισάμενος εἰπέ. τέτταρες, ὦ Σώκρατες. οὐκοῦν γένοιτʼ ἂν τούτου τοῦ χωρίου ἕτερον διπλάσιον, τοιοῦτον δέ, ἴσας ἔχον πάσας τὰς γραμμὰς ὥσπερ τοῦτο; ναί. πόσων οὖν ἔσται ποδῶν; ὀκτώ. φέρε δή, πειρῶ μοι εἰπεῖν πηλίκη τις ἔσται

But as it is two feet also on that side, it must be twice two feet? It is. Then the space is twice two feet? Yes. Well, how many are twice two feet? Count and tell me. Four, Socrates. And might there not be another figure twice the size of this, but of the same sort, with all its sides equal like this one? Yes. Then how many feet will it be? Eight. Come now, try and tell me how long will each side