Letter 865: Anger is a fire: useful when controlled, devastating when unleashed.

To Orion. The pleasures of the body are glib, and terribly skilled at bewitching, and at casting reason out of its sovereignty, and at turning everything upside down. But if the spirit [thymos, the passionate part of the soul] will fight as an ally of reason, and, like a shield-bearer obeying a king, will heed it, those pleasures will easily be vanquished. For when a just anger is stirred against them, they will not withstand the threat. Rather, they will depart and be gone, surrendering the victory to reason, to which the victory is owed. And this is precisely what the Melodist [the Psalmist David] enjoined, and what you wished to learn: "Be angry, and do not sin."

Αἱ μὲν τοῦ σώματος ἡδοναὶ στωμύλαι εἰσὶ, καὶ δειναὶ γοητεῦσαι, καὶ τὸν λογισμὸν τῆς ἡγεμονίας ἐκβάλλειν, καὶ πάντα ἄνω καὶ κάτω ποιεῖν. Ἀλλ’ εἰ ὁ θυμὸς τῷ λογισμῷ συμμαχήσει, καὶ καθάπερ ὑπασπιστὴς βασιλεῖ ὑπακούσει, ῥᾳδίως ἐκεῖνα καταγωνισθήσονται. Δίχαιας γὰρ ὀργῆς κατ’ αὐτῶν κινηθείσης οὐχ ὑποστήσονται τὴν ἀπειλήν. Ἀλλὰ φροῦδοι οἰχήσονται, τὸ νικᾶν, τῷ νικᾶν ὀφεί- λοντι λογισμῷ ἐπιτρέψασαι. Καὶ τοῦτ’ ἐστιν ὅπερ ὁ Μελῳδὸς μὲν παρεκελεύσατο, σὺ δὲ μαθεῖν ἠθέλη- σας· «Ὀργίζεσθε, καὶ μὴ ἁμαρτάνετε.»