Objection 1. It would seem that confession is not an act of the virtue of penance. For an act belongs to the virtue which is its cause. Now the cause of confession is the hope of pardon, as appears from the definition given above (Article 1). Therefore it seems that it is an act of hope and not of penance.
Objection 2. Further, shame is a part of temperance. But confession arises from shame, as appears in the definition given above (1, Objection 4). Therefore it is an act of temperance and not of penance.
Objection 3. Further, the act of penance leans on Divine mercy. But confession leans rather on Divine wisdom, by reason of the truth which is required in it. Therefore it is not an act of penance.
Objection 4. Further, we are moved to penance by the article of the Creed which is about the Judgment, on account of fear, which is the origin of penance. But we are moved to confession by the article which is about life everlasting, because it arises from hope of pardon. Therefore it is not an act of penance.
Objection 5. Further, it belongs to the virtue of truth that a man shows himself to be what he is. But this is what a man does when he goes to confession. Therefore confession is an act of that virtue which is called truth, and not of penance.
On the contrary, Penance is ordained for the destruction of sin. Now confession is ordained to this also. Therefore it is an act of penance.
I answer that, It must be observed with regard to virtues, that when a special reason of goodness or difficulty is added over and above the object of a virtue, there is need of a special virtue: thus the expenditure of large sums is the object of magnificence, although the ordinary kind of average expenditure and gifts belongs to liberality, as appears from Ethic. ii, 7; iv, 1. The same applies to the confession of truth, which, although it belongs to the virtue of truth absolutely, yet, on account of the additional reason of goodness, begins to belong to another kind of virtue. Hence the Philosopher says (Ethic. iv, 7) that a confession made in a court of justice belongs to the virtue of justice rather than to truth. In like manner the confession of God's favors in praise of God, belongs not to truth, but to religion: and so too the confession of sins, in order to receive pardon for them, is not the elicited act of the virtue of truth, as some say, but of the virtue of penance. It may, however, be the commanded act of many virtues, in so far as the act of confession can be directed to the end of many virtues.
Reply to Objection 1. Hope is the cause of confession, not as eliciting but as commanding.
Reply to Objection 2. In that definition shame is not mentioned as the cause of confession, since it is more of a nature to hinder the act of confession, but rather as the joint cause of delivery from punishment (because shame is in itself a punishment), since also the keys of the Church are the joint cause with confession, to the same effect.
Reply to Objection 3. By a certain adaptation the parts of Penance can be ascribed to three Personal Attributes, so that contrition may correspond to mercy or goodness, by reason of its being sorrow for evil--confession to wisdom, by reason of its being a manifestation of the truth--and satisfaction to power, on account of the labor it entails. And since contrition is the first part of Penance, and renders the other parts efficacious, for this reason the same is to be said of Penance as a whole, as of contrition.
Reply to Objection 4. Since confession results from hope rather than from fear, as stated above (1, ad 2), it is based on the article about eternal life which hope looks to, rather than on the article about the Judgment, which fear considers; although penance, in its aspect of contrition, is the opposite.
The Reply to the Fifth Objection is to be gathered from what has been said.
Source: Summa Theologica (New Advent)