Plato, Letters 314b

μὲν μαθεῖν, δυνατοὶ δὲ μνημονεῦσαι καὶ βασανίσαντες πάντῃ πάντως κρῖναι, γέροντες ἤδη καὶ οὐκ ἐλάττω τριάκοντα ἐτῶν ἀκηκοότες, οἳ νῦν ἄρτι σφίσι φασὶν τὰ μὲν τότε ἀπιστότατα δόξαντα εἶναι νῦν πιστότατα καὶ ἐναργέστατα φαίνεσθαι, ἃ δὲ τότε πιστότατα, νῦν τοὐναντίον. πρὸς ταῦτʼ οὖν σκοπῶν εὐλαβοῦ μή ποτέ σοι μεταμελήσῃ τῶν νῦν ἀναξίως ἐκπεσόντων. μεγίστη δὲ φυλακὴ τὸ μὴ γράφειν ἀλλʼ ἐκμανθάνειν·

who have listened to these doctrines—men capable of learning and capable also of holding them in mind and judging them by all sorts of tests—and who have been hearers of mine for no less than thirty years and are now quite old; and these men now declare that the doctrines that they once held to be most incredible appear to them now the most credible, and what they then held most credible now appears the Opposite. So, bearing this in mind, have a care lest one day you should repent of what has now been divulged improperly. The greatest safeguard is to avoid writing and to learn by heart;