St. John Cassian: Struggle against the demon of anger

Our fourth struggle is against the demon of anger. We must, with God's help, eradicate his deadly poison from the depths of our souls. So long as he dwells in our hearts and blinds the eyes of the heart with his somber disorders, we can neither discriminate what is for our good, nor achieve spiritual knowledge, nor fulfill our good intentions, nor participate in true life; and our intellect will remain impervious to the contemplation of the true, divine light; for it is written, "For my eye is troubled because of anger" (Ps 6:7 LXX). Nor will we share in divine wisdom even though we are deemed wise by all men, for it is written: "Anger lodges in the bosom of fools" (Exxles 7:9). Nor can we discriminate in decisions affecting our salvation, even though  we are thought by our fellow men to have good sense, for it is written: "Anger destroys even men of good sense" (Prov 15:1 LXX). Nor will we be able to keep our lives in righteousness with a watchful heart, for it is written: "Man's anger does not bring about the righteousness of God" (James 1:20). Nor will we be able to acquire the decorum and dignity praised by all, for it is written: "An angry man is not dignified" (Prov 11:25 LXX).
If, therefore, you desire to attain perfection and rightly to pursue the spiritual way, you should make yourself a stranger to all sinful anger and wrath. Listen to what St. Paul enjoins: "Rid yourselves of all bitterness, wrath, anger, clamor, evil speaking and all malice" (Eph 4:31). In saying 'all' he leaves no excuse for regarding any anger as necessary or reasonable. If you want to correct your brother when he is doing wrong or to punish him, you must try to keep yourself calm; otherwise you yourself may catch the sickness you are seeking to cure and you may find that the words of the Gospel now apply to you: "Physician heal yourself" (Luke 4:23), or "Why do you look at the speck of dust in your brother's eye, and not notice the rafter in your own eye?"(Matt 7:3).
+St. John Cassian, from On the Eight Vices, #4 On Anger, in The Philokalia, V 1.