Plato, Meno 84a

εἰ μὴ βούλει ἀριθμεῖν, ἀλλὰ δεῖξον ἀπὸ ποίας. ἀλλὰ μὰ τὸν Δία, ὦ Σώκρατες, ἔγωγε οὐκ οἶδα. ἐννοεῖς αὖ, ὦ Μένων, οὗ ἐστιν ἤδη βαδίζων ὅδε τοῦ ἀναμιμνῄσκεσθαι; ὅτι τὸ μὲν πρῶτον ᾔδει μὲν οὔ, ἥτις ἐστὶν ἡ τοῦ ὀκτώποδος χωρίου γραμμή, ὥσπερ οὐδὲ νῦν πω οἶδεν, ἀλλʼ οὖν ᾤετό γʼ αὐτὴν τότε εἰδέναι, καὶ θαρραλέως ἀπεκρίνετο ὡς εἰδώς, καὶ οὐχ ἡγεῖτο ἀπορεῖν· νῦν δὲ ἡγεῖται

and if you would rather not reckon it out, just show what line it is. Well, on my word, Socrates, I for one do not know. There now, Meno, do you observe who progress he has already made in his recollection? At first he did not know what is the line that forms the figure of eight feet, and he does not know even now: but at any rate he thought he knew then, and confidently answered as though he knew, and was aware of no difficulty; whereas now he feels the difficulty he is in, and besides not knowing does not think