Objection 1. It would seem that pain is not a passion of the soul. Because no passion of the soul is in the body. But pain can be in the body, since Augustine says (De Vera Relig. xii), that "bodily pain is a sudden corruption of the well-being of that thing which the soul, by making evil use of it, made subject to corruption." Therefore pain is not a passion of the soul.
Objection 2. Further, every passion of the soul belongs to the appetitive faculty. But pain does not belong to the appetitive, but rather to the apprehensive part: for Augustine says (De Nat. Boni xx) that "bodily pain is caused by the sense resisting a more powerful body." Therefore pain is not a passion of the soul.
Objection 3. Further, every passion of the soul belongs to the animal appetite. But pain does not belong to the animal appetite, but rather to the natural appetite; for Augustine says (Gen. ad lit. viii, 14): "Had not some good remained in nature, we should feel no pain in being punished by the loss of good." Therefore pain is not a passion of the soul.
On the contrary, Augustine (De Civ. Dei xiv, 8) reckons pain among the passions of the soul; quoting Virgil (Aeneid, vi, 733): "hence wild desires and grovelling fears/And human laughter, human tears." [Translation: Conington.]
I answer that, Just as two things are requisite for pleasure; namely, conjunction with good and perception of this conjunction; so also two things are requisite for pain: namely, conjunction with some evil (which is in so far evil as it deprives one of some good), and perception of this conjunction. Now whatever is conjoined, if it have not the aspect of good or evil in regard to the being to which it is conjoined, cannot cause pleasure or pain. Whence it is evident that something under the aspect of good or evil is the object of the pleasure or pain. But good and evil, as such, are objects of the appetite. Consequently it is clear that pleasure and pain belong to the appetite.
Now every appetitive movement or inclination consequent to apprehension, belongs to the intellective or sensitive appetite: since the inclination of the natural appetite is not consequent to an apprehension of the subject of that appetite, but to the apprehension of another, as stated in the I, 13, 1,3. Since then pleasure and pain presuppose some sense or apprehension in the same subject, it is evident that pain, like pleasure, is in the intellective or sensitive appetite.
Again every movement of the sensitive appetite is called a passion, as stated above (22, 1,3): and especially those which tend to some defect. Consequently pain, according as it is in the sensitive appetite, is most properly called a passion of the soul: just as bodily ailments are properly called passions of the body. Hence Augustine (De Civ. Dei xiv, 7,[8 [Quoting Cicero]) reckons pain especially as being a kind of ailment.
Reply to Objection 1. We speak of the body, because the cause of pain is in the body: as when we suffer something hurtful to the body. But the movement of pain is always in the soul; since "the body cannot feel pain unless the soul feel it," as Augustine says (Super Psalm 87:4).
Reply to Objection 2. We speak of pain of the senses, not as though it were an act of the sensitive power; but because the senses are required for bodily pain, in the same way as for bodily pleasure.
Reply to Objection 3. Pain at the loss of good proves the goodness of the nature, not because pain is an act of the natural appetite, but because nature desires something as good, the removal of which being perceived, there results the passion of pain in the sensitive appetite.
Source: Summa Theologica (New Advent)