Of God and His Creatures

It is not the present practice for the confessor regularly to impose a penance such as he reckons adequate to all the debt of temporal punishment still owing for the sins confessed. Penances comparatively light and easy are imposed for grievous sins: for the rest, the penitent is left to his own penitential efforts, to the gaining of indulgences, and to the justice of God. When confessarius primus has imposed a penance smacking of the old canonical rigour, confessarius secundus finds next time that the penance has not been performed, nor is going to be.

Of God and His Creatures: 4.72