Plato, Meno 86b

οὐκοῦν εἰ ἀεὶ ἡ ἀλήθεια ἡμῖν τῶν ὄντων ἐστὶν ἐν τῇ ψυχῇ, ἀθάνατος ἂν ἡ ψυχὴ εἴη, ὥστε θαρροῦντα χρὴ ὃ μὴ τυγχάνεις ἐπιστάμενος νῦν—τοῦτο δʼ ἐστὶν ὃ μὴ μεμνημένος—ἐπιχειρεῖν ζητεῖν καὶ ἀναμιμνῄσκεσθαι; εὖ μοι δοκεῖς λέγειν, ὦ Σώκρατες, οὐκ οἶδʼ ὅπως. καὶ γὰρ ἐγὼ ἐμοί, ὦ Μένων. καὶ τὰ μέν γε ἄλλα οὐκ ἂν πάνυ ὑπὲρ τοῦ λόγου διισχυρισαίμην· ὅτι δʼ οἰόμενοι δεῖν ζητεῖν ἃ μή τις οἶδεν βελτίους ἂν εἶμεν καὶ ἀνδρικώτεροι καὶ ἧττον ἀργοὶ ἢ εἰ οἰοίμεθα ἃ μὴ ἐπιστάμεθα μηδὲ

in our soul, then the soul must be immortal; so that you should take heart and, whatever you do not happen to know at present—that is, what you do not remember—you must endeavor to search out and recollect? What you say commends itself to me, Socrates, I know not how. And so it does to me, Meno. Most of the points I have made in support of my argument are not such as I can confidently assert; but that the belief in the duty of inquiring after what we do not know will make us better and braver and less helpless than the notion that there is not even a possibility of discovering what we do not know,