Objection 1. It would seem that contrition, confession, and satisfaction are not fittingly assigned as parts of Penance. For contrition is in the heart, and so belongs to interior penance; while confession consists of words, and satisfaction in deeds; so that the two latter belong to interior penance. Now interior penance is not a sacrament, but only exterior penance which is perceptible by the senses. Therefore these three parts are not fittingly assigned to the sacrament of Penance.
Objection 2. Further, grace is conferred in the sacraments of the New Law, as stated above (62, 1,3). But no grace is conferred in satisfaction. Therefore satisfaction is not part of a sacrament.
Objection 3. Further, the fruit of a thing is not the same as its part. But satisfaction is a fruit of penance, according to Luke 3:8: "Bring forth... fruits worthy of penance." Therefore it is not a part of Penance.
Objection 4. Further, Penance is ordained against sin. But sin can be completed merely in the thought by consent, as stated in I-II, 72, 7: therefore Penance can also. Therefore confession in word and satisfaction in deed should not be reckoned as parts of Penance.
On the contrary, It seems that yet more parts should be assigned to Penance. For not only is the body assigned as a part of man, as being the matter, but also the soul, which is his form. But the aforesaid three, being the acts of the penitent, stand as matter, while the priestly absolution stands as form. Therefore the priestly absolution should be assigned as a fourth part of Penance.
I answer that, A part is twofold, essential and quantitative. The essential parts are naturally the form and the matter, and logically the genus and the difference. In this way, each sacrament is divided into matter and form as its essential parts. Hence it has been said above (60, 5,6) that sacraments consist of things and words. But since quantity is on the part of matter, quantitative parts are parts of matter: and, in this way, as stated above (Article 1), parts are assigned specially to the sacrament of Penance, as regards the acts of the penitent, which are the matter of this sacrament.
Now it has been said above (85, 3, ad 3) that an offense is atoned otherwise in Penance than in vindictive justice. Because, in vindictive justice the atonement is made according to the judge's decision, and not according to the discretion of the offender or of the person offended; whereas, in Penance, the offense is atoned according to the will of the sinner, and the judgment of God against Whom the sin was committed, because in the latter case we seek not only the restoration of the equality of justice, as in vindictive justice, but also and still more the reconciliation of friendship, which is accomplished by the offender making atonement according to the will of the person offended. Accordingly the first requisite on the part of the penitent is the will to atone, and this is done by contrition; the second is that he submit to the judgment of the priest standing in God's place, and this is done in confession; and the third is that he atone according to the decision of God's minister, and this is done in satisfaction: and so contrition, confession, and satisfaction are assigned as parts of Penance.
Reply to Objection 1. Contrition, as to its essence, is in the heart, and belongs to interior penance; yet, virtually, it belongs to exterior penance, inasmuch as it implies the purpose of confessing and making satisfaction.
Reply to Objection 2. Satisfaction confers grace, in so far as it is in man's purpose, and it increases grace, according as it is accomplished, just as Baptism does in adults, as stated above (68, 2; 69, 8).
Reply to Objection 3. Satisfaction is a part of Penance as a sacrament, and a fruit of penance as a virtue.
Reply to Objection 4. More things are required for good, "which proceeds from a cause that is entire," than for evil, "which results from each single defect," as Dionysius states (Div. Nom. iv). And thus, although sin is completed in the consent of the heart, yet the perfection of Penance requires contrition of the heart, together with confession in word and satisfaction in deed.
The Reply to the Fifth Objection is clear from what has been said.
Source: Summa Theologica (New Advent)