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.
Satisfaction is a part of Penance as a sacrament, and a fruit of penance as a virtue.
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. 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. Further, no integral part includes within itself another that is condivided with it.