True penitence cannot ever prescind from physical ascetism. Our whole being, in fact, body and soul, (indeed the whole of nature, even animals without reason, as Holy Scripture often points out) 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. 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. On the contrary mortification aims at the "liberation" of man, who often finds himself, because of concupiscence, almost chained by his own senses. Through "corporal fasting" man regains strength and the "wound inflicted on the dignity of our nature by intemperance is cured by the medicine of a salutary abstinence.
(Papal Writings, Paenitemini, 1966)