Reply Obj. 1: By corporeal things taken in a wide sense we may understand also external sensible actions, which are to this sacrament what water is to Baptism, or chrism to Confirmation. But it is to be observed that in those sacraments, whereby an exceptional grace surpassing altogether the proportion of a human act, is conferred, some corporeal matter is employed externally, e.g. in Baptism, which confers full remission of all sins, both as to guilt and as to punishment, and in Confirmation, wherein the fulness of the Holy Ghost is bestowed, and in Extreme Unction, which confers perfect spiritual health derived from the virtue of Christ as from an extrinsic principle. Wherefore, such human acts as are in these sacraments, are not the essential matter of the sacrament, but are dispositions thereto. On the other hand, in those sacraments whose effect corresponds to that of some human act, the sensible human act itself takes the place of matter, as in the case of Penance and Matrimony, even as in bodily medicines, some are applied externally, such as plasters and drugs, while others are acts of the person who seeks to be cured, such as certain exercises.
Reply Obj. 2: In those sacraments which have a corporeal matter, this matter needs to be applied by a minister of the Church, who stands in the place of Christ, which denotes that the excellence of the power which operates in the sacraments is from Christ. But in the sacrament of Penance, as stated above (ad 1), human actions take the place of matter, and these actions proceed from internal inspiration, wherefore the matter is not applied by the minister, but by God working inwardly; while the minister furnishes the complement of the sacrament, when he absolves the penitent.
Reply Obj. 3: In Penance also, there is something which is sacrament only, viz. the acts performed outwardly both by the repentant sinner, and by the priest in giving absolution; that which is reality and sacrament is the sinner's inward repentance; while that which is reality, and not sacrament, is the forgiveness of sin. The first of these taken altogether is the cause of the second; and the first and second together are the cause of the third. _______________________
SECOND ARTICLE [III, Q. 84, Art. 2]
Whether Sins Are the Proper Matter of This Sacrament?
Objection 1: It would seem that sins are not the proper matter of this sacrament. Because, in the other sacraments, the matter is hallowed by the utterance of certain words, and being thus hallowed produces the sacramental effect. Now sins cannot be hallowed, for they are opposed to the effect of the sacrament, viz. grace which blots out sin. Therefore sins are not the proper matter of this sacrament.
Obj. 2: Further, Augustine says in his book De Poenitentia [Cf. Serm. cccli]: "No one can begin a new life, unless he repent of the old." Now not only sins but also the penalties of the present life belong to the old life. Therefore sins are not the proper matter of Penance.
Obj. 3: Further, sin is either original, mortal or venial. Now the sacrament of Penance is not ordained against original sin, for this is taken away by Baptism, [nor against mortal sin, for this is taken away by the sinner's confession]*, nor against venial sin, which is taken away by the beating of the breast and the sprinkling of holy water and the like. Therefore sins are not the proper matter of Penance. [*The words in brackets are omitted in the Leonine edition].
On the contrary, The Apostle says (2 Cor. 12:21): "(Who) have not done penance for the uncleanness and fornication and lasciviousness, that they have committed."
I answer that, Matter is twofold, viz. proximate and remote: thus the proximate matter of a statue is a metal, while the remote matter is water. Now it has been stated (A. 1, ad 1, ad 2), that the proximate matter of this sacrament consists in the acts of the penitent, the matter of which acts are the sins over which he grieves, which he confesses, and for which he satisfies. Hence it follows that sins are the remote matter of Penance, as a matter, not for approval, but for detestation, and destruction.