Objection 1. It would seem that fear is not a special passion. For Augustine says (QQ. 83, qu. 33) that "the man who is not distraught by fear, is neither harassed by desire, nor wounded by sickness"--i.e. sorrow--"nor tossed about in transports of empty joys." Wherefore it seems that, if fear be set aside, all the other passions are removed. Therefore fear is not a special but a general passion.
Objection 2. Further, the Philosopher says (Ethic. vi, 2) that "pursuit and avoidance in the appetite are what affirmation and denial are in the intellect." But denial is nothing special in the intellect, as neither is affirmation, but something common to many. Therefore neither is avoidance anything special in the appetite. But fear is nothing but a kind of avoidance of evil. Therefore it is not a special passion.
Objection 3. Further, if fear were a special passion, it would be chiefly in the irascible part. But fear is also in the concupiscible: since the Philosopher says (Rhet. ii, 5) that "fear is a kind of sorrow"; and Damascene says (De Fide Orth. iii, 23) that fear is "a power of desire": and both sorrow and desire are in the concupiscible faculty, as stated above (Question 23, Article 4). Therefore fear is not a special passion, since it belongs to different powers.
On the contrary, Fear is condivided with the other passions of the soul, as is clear from Damascene (De Fide Orth. ii, 12,15).
I answer that, The passions of the soul derive their species from their objects: hence that is a special passion, which has a special object. Now fear has a special object, as hope has. For just as the object of hope is a future good, difficult but possible to obtain; so the object of fear is a future evil, difficult and irresistible. Consequently fear is a special passion of the soul.
Reply to Objection 1. All the passions of the soul arise from one source, viz. love, wherein they are connected with one another. By reason of this connection, when fear is put aside, the other passions of the soul are dispersed; not, however, as though it were a general passion.
Reply to Objection 2. Not every avoidance in the appetite is fear, but avoidance of a special object, as stated. Wherefore, though avoidance be something common, yet fear is a special passion.
Reply to Objection 3. Fear is nowise in the concupiscible: for it regards evil, not absolutely, but as difficult or arduous, so as to be almost unavoidable. But since the irascible passions arise from the passions of the concupiscible faculty, and terminate therein, as stated above (Question 25, Article 01); hence it is that what belongs to the concupiscible is ascribed to fear. For fear is called sorrow, in so far as the object of fear causes sorrow when present: wherefore the Philosopher says (Rhet. ii, 5) that fear arises "from the representation of a future evil which is either corruptive or painful." In like manner desire is ascribed by Damascene to fear, because just as hope arises from the desire of good, so fear arises from avoidance of evil; while avoidance of evil arises from the desire of good, as is evident from what has been said above (25, 2; 29, 02; 36, 2).
Source: Summa Theologica (New Advent)