Objection 1. It would seem that penance is not a special virtue. For it seems that to rejoice at the good one has done, and to grieve for the evil one has done are acts of the same nature. But joy for the good one has done is not a special virtue, but is a praiseworthy emotion proceeding from charity, as Augustine states (De Civ. Dei xiv, 7,8,9): wherefore the Apostle says (1 Corinthians 13:6) that charity "rejoiceth not at iniquity, but rejoiceth with the truth." Therefore, in like manner, neither is penance, which is sorrow for past sins, a special virtue, but an emotion resulting from charity.
Objection 2. Further, every special virtue has its special matter, because habits are distinguished by their acts, and acts by their objects. But penance has no special matter, because its matter is past sins in any matter whatever. Therefore penance is not a special virtue.
Objection 3. Further, nothing is removed except by its contrary. But penance removes all sins. Therefore it is contrary to all sins, and consequently is not a special virtue.
On the contrary, The Law has a special precept about penance, as stated above (84, 5,7).
I answer that, As stated in I-II, 54, 1, ad 1, 2, habits are specifically distinguished according to the species of their acts, so that whenever an act has a special reason for being praiseworthy, there must needs be a special habit. Now it is evident that there is a special reason for praising the act of penance, because it aims at the destruction of past sin, considered as an offense against God, which does not apply to any other virtue. We must therefore conclude that penance is a special virtue.
Reply to Objection 1. An act springs from charity in two ways: first as being elicited by charity, and a like virtuous act requires no other virtue than charity, e.g. to love the good, to rejoice therein, and to grieve for what is opposed to it. Secondly, an act springs from charity, being, so to speak, commanded by charity; and thus, since charity commands all the virtues, inasmuch as it directs them to its own end, an act springing from charity may belong even to another special virtue. Accordingly, if in the act of the penitent we consider the mere displeasure in the past sin, it belongs to charity immediately, in the same way as joy for past good acts; but the intention to aim at the destruction of past sin requires a special virtue subordinate to charity.
Reply to Objection 2. In point of fact, penance has indeed a general matter, inasmuch as it regards all sins; but it does so under a special aspect, inasmuch as they can be remedied by an act of man in co-operating with God for his justification.
Reply to Objection 3. Every special virtue removes formally the habit of the opposite vice, just as whiteness removes blackness from the same subject: but penance removes every sin effectively, inasmuch as it works for the destruction of sins, according as they are pardonable through the grace of God if man co-operate therewith. Wherefore it does not follow that it is a general virtue.
Source: Summa Theologica (New Advent)