Objection 1. It seems that the brave man does not use anger in his action. For no one should employ as an instrument of his action that which he cannot use at will. Now man cannot use anger at will, so as to take it up and lay it aside when he will. For, as the Philosopher says (De Memoria ii), when a bodily passion is in movement, it does not rest at once just as one wishes. Therefore a brave man should not employ anger for his action.
Objection 2. Further, if a man is competent to do a thing by himself, he should not seek the assistance of something weaker and more imperfect. Now the reason is competent to achieve by itself deeds of fortitude, wherein anger is impotent: wherefore Seneca says (De Ira i): "Reason by itself suffices not only to make us prepared for action but also to accomplish it. On fact is there greater folly than for reason to seek help from anger? the steadfast from the unstaid, the trusty from the untrustworthy, the healthy from the sick?" Therefore a brave man should not make use of anger.
Objection 3. Further, just as people are more earnest in doing deeds of fortitude on account of anger, so are they on account of sorrow or desire; wherefore the Philosopher says (Ethic. iii, 8) that wild beasts are incited to face danger through sorrow or pain, and adulterous persons dare many things for the sake of desire. Now fortitude employs neither sorrow nor desire for its action. Therefore in like manner it should not employ anger.
On the contrary, The Philosopher says (Ethic. iii, 8) that "anger helps the brave."
I answer that, As stated above (I-II, 24, 2), concerning anger and the other passions there was a difference of opinion between the Peripatetics and the Stoics. For the Stoics excluded anger and all other passions of the soul from the mind of a wise or good man: whereas the Peripatetics, of whom Aristotle was the chief, ascribed to virtuous men both anger and the other passions of the soul albeit modified by reason. And possibly they differed not in reality but in their way of speaking. For the Peripatetics, as stated above (I-II, 24, 2), gave the name of passions to all the movements of the sensitive appetite, however they may comport themselves. And since the sensitive appetite is moved by the command of reason, so that it may cooperate by rendering action more prompt, they held that virtuous persons should employ both anger and the other passions of the soul, modified according to the dictate of reason. On the other hand, the Stoics gave the name of passions to certain immoderate emotions of the sensitive appetite, wherefore they called them sicknesses or diseases, and for this reason severed them altogether from virtue.
Accordingly the brave man employs moderate anger for his action, but not immoderate anger.
Reply to Objection 1. Anger that is moderated in accordance with reason is subject to the command of reason: so that man uses it at his will, which would not be the case were it immoderate.
Reply to Objection 2. Reason employs anger for its action, not as seeking its assistance, but because it uses the sensitive appetite as an instrument, just as it uses the members of the body. Nor is it unbecoming for the instrument to be more imperfect than the principal agent, even as the hammer is more imperfect than the smith. Moreover, Seneca was a follower of the Stoics, and the above words were aimed by him directly at Aristotle.
Reply to Objection 3. Whereas fortitude, as stated above (Article 6), has two acts, namely endurance and aggression, it employs anger, not for the act of endurance, because the reason by itself performs this act, but for the act of aggression, for which it employs anger rather than the other passions, since it belongs to anger to strike at the cause of sorrow, so that it directly cooperates with fortitude in attacking. On the other hand, sorrow by its very nature gives way to the thing that hurts; though accidentally it helps in aggression, either as being the cause of anger, as stated above (I-II, 47, 3), or as making a person expose himself to danger in order to escape from sorrow. On like manner desire, by its very nature, tends to a pleasurable good, to which it is directly contrary to withstand danger: yet accidentally sometimes it helps one to attack, in so far as one prefers to risk dangers rather than lack pleasure. Hence the Philosopher says (Ethic. iii, 5): "Of all the cases in which fortitude arises from a passion, the most natural is when a man is brave through anger, making his choice and acting for a purpose," i.e. for a due end; "this is true fortitude."
Source: Summa Theologica (New Advent)