Chapter 19 — Part II: The Sacraments — Penance
Section 30. The Matter — There is nothing that should be better known to the faithful than the matter of this Sacrament; hence they should be taught that Penance differs from the other Sacraments in this that while the matter of the other Sacraments is some thing, whether natural or artificial, the matter, as it were, of the Sacrament of Penance is the acts of the penitent, -- namely, contrition, confession and satisfaction, -- as has been declared by the council of Trent. Now, inasmuch as these acts are by divine institution required on the part of the penitent for the integrity of the Sacrament, and for the full and perfect remission of sin, they are called parts of Penance. It is not because they are not the real matter that they are called by the Council the matter as it were, but because they are not of that sort of matter which is applied externally, such, for instance, as water in Baptism and chrism in Confirmation.