Meditations 12.6.1
Marcus Aurelius to himself
Ἔθιζε καὶ ὅσα ἀπογινώσκεις. καὶ γὰρ ἡ χεὶρ ἡ ἀριστερὰ πρὸς τὰ ἄλλα διὰ τὸ ἀνέθιστον ἀργὸς οὖσα τοῦ χαλινοῦ ἐρρωμενέστερον ἢ ἡ δεξιὰ κρατεῖ· τοῦτο γὰρ εἴθισται.
Practice even the things you despair of. The left hand, though idle at other tasks through lack of practice, holds the rein more strongly than the right, because it has been accustomed to this.