Plato, Letters 313b

καὶ εἶναι σὸν εὕρημα· καὶ ἐγὼ εἶπον ὅτι τοῦτο εἰ φαίνοιτό σοι οὕτως ἔχειν, πολλῶν ἂν εἴης λόγων ἐμὲ ἀπολελυκώς. οὐ μὴν ἄλλῳ γέ ποτʼ ἔφην ἐντετυχηκέναι τοῦθʼ ηὑρηκότι, ἀλλὰ ἡ πολλή μοι πραγματεία περὶ τοῦτʼ εἴη· σὺ δὲ ἴσως μὲν ἀκούσας του, τάχα δʼ ἂν θείᾳ μοίρᾳ κατὰ τοῦθʼ ὁρμήσας, ἔπειτα αὐτοῦ τὰς ἀποδείξεις ὡς ἔχων βεβαίως οὐ κατέδησας, ἀλλʼ ᾄττει σοι τοτὲ μὲν οὕτως, τοτὲ δὲ ἄλλως

and I made answer that if it was plain to you that this was so, you would have saved me from a long discourse. I said, however, that I had never met with any other person who had made this discovery; on the contrary most of the trouble I had was about this very problem. So then, after you had either, as is probable, got the true solution from someone else, or had possibly (by Heaven’s favor) hit on it yourself, you fancied you had a firm grip on the proofs of it, and so you omitted to make them fast; thus your view of the truth sways now this way, now that, round about the apparent object; whereas the true object is wholly different.