Plato, Meno 96c

εἰ δέ γε μὴ διδάσκαλοι, οὐδὲ μαθηταί; δοκεῖ μοι ἔχειν ὡς λέγεις. ὡμολογήκαμεν δέ γε, πράγματος οὗ μήτε διδάσκαλοι μήτε μαθηταὶ εἶεν, τοῦτο μηδὲ διδακτὸν εἶναι; ὡμολογήκαΜΕΝ. οὐκοῦν ἀρετῆς οὐδαμοῦ φαίνονται διδάσκαλοι; ἔστι ταῦτα. εἰ δέ γε μὴ διδάσκαλοι, οὐδὲ μαθηταί; φαίνεται οὕτως. ἀρετὴ ἄρα οὐκ ἂν εἴη διδακτόν;

And if there are no teachers, there can be no disciples either? I think that statement is true. And we have admitted that a thing of which there are neither teachers nor disciples cannot be taught? We have. So nowhere are any teachers of virtue to be found? That is so. And if no teachers, then no disciples? So it appears. Hence virtue cannot be taught?