Objection 1. It would seem that anger is not a special passion. For the irascible power takes its name from anger [ira]. But there are several passions in this power, not only one. Therefore anger is not one special passion.
Objection 2. Further, to every special passion there is a contrary passion; as is evident by going through them one by one. But no passion is contrary to anger, as stated above (Question 23, Article 3). Therefore anger is not a special passion.
Objection 3. Further, one special passion does not include another. But anger includes several passions: since it accompanies sorrow, pleasure, and hope, as the Philosopher states (Rhet. ii, 2). Therefore anger is not a special passion.
On the contrary, Damascene (De Fide Orth. ii, 16) calls anger a special passion: and so does Cicero (De Quaest. Tusc. iv, 7).
I answer that, A thing is said to be general in two ways. First, by predication; thus "animal" is general in respect of all animals. Secondly, by causality; thus the sun is the general cause of all things generated here below, according to Dionysius (Div. Nom. iv). Because just as a genus contains potentially many differences, according to a likeness of matter; so an efficient cause contains many effects according to its active power. Now it happens that an effect is produced by the concurrence of various causes; and since every cause remains somewhat in its effect, we may say that, in yet a third way, an effect which is due to the concurrence of several causes, has a certain generality, inasmuch as several causes are, in a fashion, actually existing therein.
Accordingly in the first way, anger is not a general passion but is condivided with the other passions, as stated above (Question 23, Article 4). In like manner, neither is it in the second way: since it is not a cause of the other passions. But in this way, love may be called a general passion, as Augustine declares (De Civ. Dei xiv, 7,9), because love is the primary root of all the other passions, as stated above (Question 27, Article 4). But, in a third way, anger may be called a general passion, inasmuch as it is caused by a concurrence of several passions. Because the movement of anger does not arise save on account of some pain inflicted, and unless there be desire and hope of revenge: for, as the Philosopher says (Rhet. ii, 2), "the angry man hopes to punish; since he craves for revenge as being possible." Consequently if the person, who inflicted the injury, excel very much, anger does not ensue, but only sorrow, as Avicenna states (De Anima iv, 6).
Reply to Objection 1. The irascible power takes its name from "ira" [anger], not because every movement of that power is one of anger; but because all its movements terminate in anger; and because, of all these movements, anger is the most patent.
Reply to Objection 2. From the very fact that anger is caused by contrary passions, i.e. by hope, which is of good, and by sorrow, which is of evil, it includes in itself contrariety: and consequently it has no contrary outside itself. Thus also in mixed colors there is no contrariety, except that of the simple colors from which they are made.
Reply to Objection 3. Anger includes several passions, not indeed as a genus includes several species; but rather according to the inclusion of cause and effect.
Source: Summa Theologica (New Advent)