Letter 850: Virtue must be practiced with all one's strength — not merely admired from a distance.

To Paulos. On the saying: "He who looks at a woman." [Matthew 5:28] Since loving desire is born from seeing, for this reason Christ also declared the man who feasts his eyes unchastely to be an adulterer, checking beforehand not the act alone but the very thought as well. And in the matter of murder too, the law punishes the manslayer, but Christ punishes the man who is angry. For the one legislates for the soul, the other for the hand; and by not permitting the root to grow, he cuts off the branches in advance as well. Bridle, therefore, both the eye and your temper [thymos, the seat of anger], so that the one may not lead you by the hand into adultery, nor the other into murder.

Εἰς τὸ ῥητόν· «Ὁ βλέπων γυναῖκα.» Ἐπειδὴ ἐκ τοῦ ὁρᾶν τὸ ἐρᾷν τίκτεται, διὰ τοῦτο καὶ ὁ Χριστὸς τὸν ἀκολάστως ἐσθίοντα τὰς ὄψεις μοιχὸν εἶναι ἀπεφήνατο, οὐ τὴν πρᾶξιν μόνον, ἀλλὰ καὶ τὴν ἔννοιαν προαναστέλλων, καὶ ἐπὶ τοῦ φόνου δέ, ὁ μὲν νόμος τὸν ἀνδροφόνον, ὁ δὲ Χριστὸς τὸν ὀργιζόμενον τιμωρεῖται. Ὁ μὲν γὰρ τῇ ψυχῇ, ὁ δὲ τῇ χειρὶ νομοθετεῖ· καὶ διὰ τὸ μὴ συγχω- ρῆσαι φῦναι τὴν ῥίζαν, καὶ τοὺς κλάδους προανα- στέλλει. Χαλίνου τοίνυν καὶ τὸν ὀφθαλμὸν καὶ τὸν θυμόν, ἵνα μὴ ὁ μὲν εἰς μοιχείαν, ὁ δὲ εἰς φόνον σε χειραγωγήσῃ.