Epictetus, Works, book 1, Περὶ τῶν ἐφ’ ἡμῖν καὶ οὐκ ἐφ’ ἡμῖν. 3:

αὐτὴ οὖν αὑτὴν θεωρεῖ τισ αὐτῶν; οὐδαμῶσ. ἀλλ’ ὅτε μέν, ἄν τι γράφῃσ τῷ ἑταίρῳ, δεῖ τούτων τῶν γραπτέων, ἡ γραμματικὴ ἐρεῖ· πότερον δὲ γραπτέον τῷ ἑταίρῳ ἢ οὐ γραπτέον, ἡ γραμματικὴ οὐκ ἐρεῖ. καὶ περὶ τῶν μελῶν ὡσαύτωσ ἡ μουσική· πότερον δ’ ᾀστέον νῦν καὶ κιθαριστέον ἢ οὔτε ᾀστέον οὔτε κιθαριστέον οὐκ ἐρεῖ.