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 (Q. 60, AA. 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 (A. 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 (Q. 85, A. 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 Obj. 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 Obj. 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 (Q. 68, A. 2; Q. 69, A. 8).
Reply Obj. 3: Satisfaction is a part of Penance as a sacrament, and a fruit of penance as a virtue.
Reply Obj. 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. _______________________
THIRD ARTICLE [III, Q. 90, Art. 3]
Whether These Three Are Integral Parts of Penance?
Objection 1: It would seem that these three are not integral parts of Penance. For, as stated above (Q. 84, A. 3), Penance is ordained against sin. But sins of thought, word, and deed are the subjective and not integral parts of sin, because sin is predicated of each one of them. Therefore in Penance also, contrition in thought, confession in word, and satisfaction in deed are not integral parts.