Do not think that what is hard for you to master is humanly impossible; but if a thing is humanly possible, consider it to be within your reach.
Μή, εἴ τι αὐτῷ σοὶ δυσκαταπόνητον, τοῦτο ἀνθρώπῳ ἀδύνατον ὑπολαμβάνειν, ἀλλ εἴ τι ἀνθρώπῳ δυνατὸν καὶ οἰκεῖον, τοῦτο καὶ σεαυτῷ ἐφικτὸν νομίζειν.
Original text (Ancient Greek)