Objection 1. It would seem that the definition of satisfaction given in the text (Sent. iv, D, 15) and quoted from Augustine [Gennadius Massiliensis, De Eccl. Dogm. liv] is unsuitable--viz. that "satisfaction is to uproot the causes of sins, and to give no opening to the suggestions thereof." For the cause of actual sin is the fomes. ["Fomes" signifies literally "fuel," and metaphorically, "incentive." As used by the theologian, it denotes the quasi-material element and effect of original sin, and sometimes goes under the name of "concupiscence." [Cf. I-II, 82, 3]. But we cannot remove the "fomes" in this life. Therefore satisfaction does not consist in removing the causes of sins.
Objection 2. Further, the cause of sin is stronger than sin itself. But man by himself cannot remove sin. Much less therefore can he remove the cause of sin; and so the same conclusion follows.
Objection 3. Further, since satisfaction is a part of Penance, it regards the past and not the future. Now "to give no opening to the suggestions of sin" regards the future. Therefore it should not be put in the definition of satisfaction.
Objection 4. Further, satisfaction regards a past offense. Yet no mention is made of this. Therefore the definition of satisfaction is unsuitable.
Objection 5. Further, Anselm gives another definition (Cur Deus homo i): "Satisfaction consists in giving God due honor," wherein no reference is made to the things mentioned by Augustine [Gennadius, Objection 1 in this definition. Therefore one or the other is unsuitable.
Objection 6. Further, an innocent man can give due honor to God: whereas satisfaction is not compatible with innocence. Therefore Anselm's definition is faulty.
I answer that, Justice aims not only at removing inequality already existing, by punishing the past fault, but also at safeguarding equality for the future, because according to the Philosopher (Ethic. ii, 3) "punishments are medicinal." Wherefore satisfaction which is the act of justice inflicting punishment, is a medicine healing past sins and preserving from future sins: so that when one man makes satisfaction to another, he offers compensation for the past, and takes heed for the future. Accordingly satisfaction may be defined in two ways, first with regard to past sin, which it heals by making compensation, and thus it is defined as "compensation for an inflicted injury according to the equality of justice." The definition of Anselm amounts to the same, for he says that "satisfaction consists in giving God due honor"; where duty is considered in respect of the sin committed. Secondly, satisfaction may be defined, considered as preserving us from future sins; and as Augustine (Cf. Objection 1) defines it. Now preservation from bodily sickness is assured by removing the causes from which the sickness may ensue, for if they be taken away the sickness cannot follow. But it is not thus in spiritual diseases, for the free-will cannot be forced, so that even in the presence of their causes, they can, though with difficulty, be avoided, while they can be incurred even when their causes are removed. Hence he puts two things in the definition of satisfaction, viz. removal of the causes, as to the first, and the free-will's refusal to sin.
Reply to Objection 1. By "causes" we must understand the proximate causes of actual sin, which are twofold: viz. the lust of sin through the habit or act of a sin that has been given up, and those things which are called the remnants of past sin; and external occasions of sin, such as place, bad company and so forth. Such causes are removed by satisfaction in this life, albeit the "fomes," which is the remote cause of actual sin, is not entirely removed by satisfaction in this life though it is weakened.
Reply to Objection 2. Since the cause of evil or of privation (according as it has a cause) is nothing else than a defective good, and since it is easier to destroy good than to set it up, it follows that it is easier to uproot the causes of privation and of evil than to remove the evil itself, which can only be removed by setting up good, as may be seen in the case of blindness and its causes. Yet the aforesaid are not sufficient causes of sin, for sin does not, of necessity, ensue therefrom, but they are occasions of sin. Nor again can satisfaction be made without God's help, since it is not possible without charity, as we shall state further on (14, 2).
Reply to Objection 3. Although Penance was primarily instituted and intended with a view to the past, yet, as a consequence, it regards the future, in so far as it is a safeguarding remedy; and the same applies to satisfaction.
Reply to Objection 4. Augustine [Gennadius Massiliensis, De Eccl. Dogm. liv] defined satisfaction, as made to God, from Whom, in reality, nothing can be taken, though the sinner, for his own part, takes something away. Consequently in such like satisfaction, amendment for future time is of greater weight than compensation for the past. Hence Augustine defines satisfaction from this point of view. And yet it is possible to gauge the compensation for the past from the heed taken for the future, for the latter regards the same object as the former, but in the opposite way: since when looking at the past we detest the causes of sins on account of the sins themselves, which are the starting-point of the movement of detestation: whereas when taking heed of the future, we begin from the causes, that by their removal we may avoid sins the more easily.
Reply to Objection 5. There is no reason why the same thing should not be described in different ways according to the various things found in it: and such is the case here, as explained above.
Reply to Objection 6. By debt is meant the debt we owe to God by reason of the sins we have committed, because Penance regards a debt, as stated above (Article 2).
Source: Summa Theologica (New Advent)