Objection 1. It would seem that the works of satisfaction are unsuitably enumerated by saying that there are three, viz. almsdeeds, fasting, and prayer. For a work of satisfaction should be penal. But prayer is not penal, since it is a remedy against penal sorrow, and is a source of pleasure, wherefore it is written (James 5:13): "Is any of you sad? Let him pray. Is he cheerful in mind? Let him sing." Therefore prayer should not be reckoned among the works of satisfaction.
Objection 2. Further, every sin is either carnal or spiritual. Now, as Jerome says on Mark 9:28, "This kind" of demons "can go out by nothing, but by prayer and fasting: Diseases of the body are healed by fasting, diseases of the mind, by prayer." Therefore no other work of satisfaction is necessary.
Objection 3. Further, satisfaction is necessary in order for us to be cleansed from our sins. But almsgiving cleanses from all sins, according to Luke 11:41: "Give alms, and behold all things are clean unto you." Therefore the other two are in excess.
Objection 4. On the other hand, it seems that there should be more. For contrary heals contrary. But there are many more than three kinds of sin. Therefore more works of satisfaction should be enumerated.
Objection 5. Further, pilgrimages and scourgings are also enjoined as works of satisfaction, and are not included among the above. Therefore they are not sufficiently enumerated.
I answer that, Satisfaction should be of such a nature as to involve something taken away from us for the honor of God. Now we have but three kinds of goods, bodily, spiritual, and goods of fortune, or external goods. By alms-deeds we deprive ourselves of some goods of fortune, and by fasting we retrench goods of the body. As to goods of the soul, there is no need to deprive ourselves of any of them, either in whole or in part, since thereby we become acceptable to God, but we should submit them entirely to God, which is done by prayer.
This number is shown to be suitable in so far as satisfaction uproots the causes of sin, for these are reckoned to be three (1 John 2:16), viz. "concupiscence of the flesh," "concupiscence of the eyes," and "pride of life." Fasting is directed against concupiscence of the "flesh," alms-deeds against concupiscence of the "eyes," and "prayer" against "pride of life," as Augustine says (Enarr. in Ps. 42).
This number is also shown to be suitable in so far as satisfaction does not open a way to the suggestions of sin, because every sin is committed either against God, and this is prevented by "prayer," or against our neighbor, and this is remedied by "alms-deeds," or against ourselves, and this is forestalled by "fasting."
Reply to Objection 1. According to some, prayer is twofold. There is the prayer of contemplatives whose "conversation is in heaven": and this, since it is altogether delightful, is not a work of satisfaction. The other is a prayer which pours forth sighs for sin; this is penal and a part of satisfaction.
It may also be replied, and better, that every prayer has the character of satisfaction, for though it be sweet to the soul it is painful to the body, since, as Gregory says (Super Ezech., Hom. xiv), "doubtless, when our soul's love is strengthened, our body's strength is weakened"; hence we read (Genesis 32:25) that the sinew of Jacob's thigh shrank through his wrestling with the angel.
Reply to Objection 2. Carnal sin is twofold; one which is completed in carnal delectation, as gluttony and lust. and, another which is completed in things relating to the flesh, though it be completed in the delectation of the soul rather than of the flesh, as covetousness. Hence such like sins are between spiritual and carnal sins, so that they need a satisfaction proper to them, viz. almsdeeds.
Reply to Objection 3. Although each of these three, by a kind of likeness, is appropriated to some particular kind of sin because it is reasonable that, whereby a man sins, in that he should be punished, and that satisfaction should cut out the very root of the sin committed, yet each of them can satisfy for any kind of sin. Hence if a man is unable to perform one of the above, another is imposed on him, chiefly almsdeeds, which can take the place of the others, in so far as in those to whom a man gives alms he purchases other works of satisfaction thereby. Consequently even if almsgiving washes all sins away, it does not follow that other works are in excess.
Reply to Objection 4. Though there are many kinds of sins, all are reduced to those three roots or to those three kinds of sin, to which, as we have said, the aforesaid works of satisfaction correspond.
Reply to Objection 5. Whatever relates to affliction of the body is all referred to fasting, and whatever is spent for the benefit of one's neighbor is a kind of alms, and whatever act of worship is given to God becomes a kind of prayer, so that even one work can be satisfactory in several ways.
Source: Summa Theologica (New Advent)