The following is an excerpt from the Apostolic Constitution Paenitemini of St. Paul VI of 1966

True penitence, however, cannot ever prescind from physical asceticism as well. Our whole being in fact, body and soul, (indeed the whole of nature, even animals without reason, as Holy Scripture often points out)(46) must participate actively in this religious act whereby the creature recognizes divine holiness and majesty.

The necessity of the mortification of the flesh also stands clearly revealed if we consider the fragility of our nature, in which, since Adam’s sin, flesh and spirit have contrasting desires.(47) This exercise of bodily mortification-far removed from any form of stoicism does not imply a condemnation of the flesh which sons of God deign to assume.(48) On the contrary, mortification aims at the “liberation“(49) of man, who often finds himself, because of concupiscence, almost chained(50) by his own senses.

Through “corporal fasting“(51) man regains strength and the “wound inflicted on the dignity of our nature by intemperance is cured by the medicine of a salutary abstinence.“(52)

Nevertheless, in the New Testament and in the history of the Church—although the duty of doing penance is motivated above all by participation in the sufferings of Christ-the necessity of an asceticism which chastises the body and brings it into subjection is affirmed with special insistence by the example of Christ Himself.(53)
Against the real and ever recurring danger of formalism and pharisaism the Divine Master in the New Covenant openly condemned—and so have the Apostles, Fathers and supreme pontiffs—any form of penitence which is purely external.

The intimate relationship which exists in penitence between the external act, inner conversion, prayer and works of charity is affirmed and widely developed in the liturgical texts and authors of every era.(54)