Thomas Aquinas

“ Parts are not assigned to penance as a virtue: because the human acts of which there are several in penance, are related to the habit of virtue, not as its parts, but as its effects. It follows, therefore, that parts are assigned to Penance as a sacrament, to which the human acts are related as matter: whereas in the other sacraments the matter does not consist of human acts, but of some one external thing, either simple, as water or oil, or compound, as chrism, and so parts are not assigned to the other sacraments. ”

“ Another species is the sin that is completed in thought and word: and yet a third species is the sin that is completed in thought, word, and deed; and the quasi-integral parts of this last sin, are that which is in thought, that which is in word, and that which is in deed. Wherefore these three are the integral parts of Penance, which is completed in them.
Reply to Objection 2: One integral part can include the whole, though not as to its essence: because the foundation, in a way, contains virtually the whole building. In this way contrition includes virtually the whole of Penance. ”

“ Every sacrament is something simple by reason of the Divine power, which operates therein: but the Divine power is so great that it can operate both through one and through many, and by reason of these many, parts may be assigned to a particular sacrament. ”