Epictetus, Works, book 4, Περὶ ἐλευθερίασ. 63:

τί οὖν ἐστι τὸ ἐν τῷ γράφειν ἀκώλυτον ποιοῦν καὶ ἀπαραπόδιστον; ‐ Ἡ ἐπιστήμη τοῦ γράφειν. ‐ Τί δ’ ἐν τῷ κιθαρίζειν; ‐ Ἡ ἐπιστήμη τοῦ κιθαρίζειν. ‐ Οὐκοῦν καὶ ἐν τῷ βιοῦν ἡ ἐπιστήμη τοῦ βιοῦν.