Plato, Laws 791c

ἐπιτήδευμα εἶναι, τὸ νικᾶν τὰ προσπίπτονθʼ ἡμῖν δείματά τε καὶ φόβους. ὀρθῶς. ἓν δὴ καὶ τοῦτο εἰς ψυχῆς μόριον ἀρετῆς, τὴν τῶν παντελῶς παίδων γυμναστικὴν ἐν ταῖς κινήσεσιν, μέγα ἡμῖν φῶμεν συμβάλλεσθαι. πάνυ μὲν οὖν. καὶ μὴν τό γε μὴ δύσκολον ἐν ψυχῇ καὶ τὸ δύσκολον οὐ σμικρὸν μόριον εὐψυχίας καὶ κακοψυχίας ἑκάτερον γιγνόμενον γίγνοιτʼ ἄν. πῶς δʼ οὔ;

The opposite course, of practicing courage from youth up, consists, we shall say, in the conquering of the frights and fears that assail us. That is true. Let us say, then, that this factor—namely, the exercise of quite young children by the various motions—contributes greatly towards developing one part of the soul’s virtue. Certainly. Moreover, cheerfulness of soul and its opposite will constitute no small part of stoutheartedness and faintheartedness. Of course.