Chapter 19 — Part II: The Sacraments — Penance
Section 31. As regards the opinion of some who hold that sins themselves are the matter of this Sacrament, it will be found, when carefully examined, that it does not really differ from the explanation already given. Thus we say that wood which is consumed by fire is the matter of fire. In the same way, sins which are destroyed by Penance may properly be called the matter of Penance.