Plato, Letters 341a

ἐπιτηδεύειν δυνατοὶ γίγνονται, ἔνιοι δὲ αὐτῶν πείθουσιν αὑτοὺς ὡς ἱκανῶς ἀκηκοότες εἰσὶν τὸ ὅλον, καὶ οὐδὲν ἔτι δέονταί τινων πραγμάτων. ἡ μὲν δὴ πεῖρα αὕτη γίγνεται ἡ σαφής τε καὶ ἀσφαλεστάτη πρὸς τοὺς τρυφῶντάς τε καὶ ἀδυνάτους διαπονεῖν, ὡς μηδέποτε βαλεῖν ἐν αἰτίᾳ τὸν δεικνύντα ἀλλʼ αὐτὸν αὑτόν, μὴ δυνάμενον πάντα τὰ πρόσφορα ἐπιτηδεύειν τῷ πράγματι. οὕτω δὴ καὶ Διονυσίῳ τότʼ ἐρρήθη τὰ ῥηθέντα. πάντα μὲν οὖν οὔτʼ ἐγὼ διεξῆλθον οὔτε

while some of them persuade themselves that they have been sufficiently instructed in the whole subject and no longer require any further effort. Now this test proves the clearest and most infallible in dealing with those who are luxurious and incapable of enduring labor, since it prevents any of them from ever casting the blame on his instructor instead of on himself and his own inability to pursue all the studies which are accessory to his subject. This, then, was the purport of what I said to Dionysius on that occasion. I did not, however, expound the matter fully, nor did Dionysius ask me to do so;