Seneca considers the argument that clemency enables wicked people, and replies that while excessive pardoning of faults is vicious, nonetheless we should regard clemency itself as a medicinal virtue. The healthy do not despise medicine, but recognize its value. And nobody is so healthy as to be able to rule out that they might need medicine some day. Clemency works very much the same way. One does not give out medicine indiscriminately; one carefully proportions the medicinal treatment to the sickness at hand. In addition, clemency is not all about saving the guilty from punishment; it saves the innocent from punishment, as well. What is more, clemency is desirable simply in itself, since it is perhaps the most human virtue, in part because it is one we tend to admire, and in part because it is highly concerned with common good. One alleviates punishment and penalty because it is sometimes, perhaps often, good for us all. Without it governments would seem harsh and demanding; with it, they are, as we say, humanized. And this is true quite broadly. Seneca points out that while gods are gods regardless of what we think of them, nobody wants gods who are inclement, and he urges the young Nero to treat his subjects with at least the clemency he wishes the gods to show him.
A prince inflicts punishment for one of two reasons, to avenge himself or to avenge another, and vengeance or vindication serves as either a compensation or a deterrent. Because of anger, it is more difficult to be clement in punishing on one's own behalf, which is why it is the standard for clemency. The clement person remits punishment even where the harm was committed against himself. Princes in particular should forego punishing another for compensation on their own behalf; and clemency may sometimes itself do as much good for future security as deterrent punishment. This brings us to punishment on the behalf of others:
Let us, pass now to the injuries done to others, in the punishment of which these three aims, which the law has had in view, should be kept in view also by the prince: either to reform the man that is punished, or by punishing him to make the rest better, or by removing bad men to let the rest live in greater security. You will more easily reform the culprits themselves by the lighter form of punishment; for he will live more guardedly who has something left to lose. No one is sparing of a ruined reputation; it brings a sort of exemption from punishment to have no room left for punishment. The morals of the state, moreover, are better mended by the sparing use of punitive measures; for sin becomes familiar from the multitude of those who sin, and the official stigma is less weighty if its force is weakened by the very number that it condemns, and severity, which provides the best corrective, loses its potency by repeated application. Good morals are established in the state and vice is wiped out if a prince is patient with vice, not as if he approved of it, but as if unwillingly and with great pain he had resort to chastisement.
Thus reform and general deterrence are often best served by a clement approach to punishment. One of Seneca's additional arguments is interesting. People who break the law or do something unjust are quite common. One of the things a state must do is to make sure it is entirely unclear how common it is. If you mark off the lawbreakers too sharply, everyone can start to count their number, and once it becomes clear from the punishment that large numbers of people are breaking the law, people who otherwise would not break the law will start to do so. If nobody's guilt is pardoned, this does not, contrary to what we might expect, make people obey the law; it makes people think that, since there's a good chance they will be punished for the inevitable slip-up, no matter how good they are the rest of the time, they might as well not worry about what the law says, and instead see what they can actually get away with.
The opposite of clementia is crudelitas, that is, cruelty, "inclination of the mind toward the side of harshness [atrocitas]". Cruelty is the most inhuman of vices; it transgresses not just ordinary bounds of human decency, but humanity itself, since it begins to search for ways to torture and begins to take delight in the suffering of others. We fall into cruelty by mistaking it for strictness. Likewise, we fall into compassion [misericordia], which Seneca as a Stoic regards as a vice, by mistaking it for clemency. As he puts it, misericordia regards only the (mis)fortune and distress, not the cause of it; whereas clementia must be rational. He recognizes that people tend not to like this teaching of the Stoic school, but insists that this is because they confuse feeling sorrow for another, which merely spreads the distress around, with actually doing something about it: the Stoic sage is motivated not by the distress of others but by the opportunity genuinely to help them. Because of this, Seneca insists that clemency is not concerned with pardon, because pardon [venia] is remission of punishment that is deserved [poenae meritae remissio].
Aquinas, when he considers clemency (ST 2-2.157), links it with the virtue of meekness. The two are distinct, but they are related in that meekness restrains the desire to vindicate while clemency restrains the external expression of it. Both are potential parts of temperance; that is, they are temperance in a broad sense, but they are distinct from temperance strictly speaking. The association with temperance is explicitly derived from Seneca, who is Aquinas's most quoted authority on the subject. It distinguishes clemency from equity, which also mitigates punishment, because equity mitigates punishment in order to preserve justice, whereas clemency does so out of "sweetness of soul". The reason clemency is a virtue is because moral virtues are simply the ordering of desire by reason, and, as Seneca says, clemency in mitigating punishment is rational.
For purely Christian reasons, however, Aquinas has to seriously consider the possibility that meekness and clemency are the greatest virtues. He ends up denying that they are in any strict sense, because both are concerned with mitigating evil, and the greatest virtues need to be concerned with positive good. However, among virtues that mitigate and restrain evil, they are the most excellent. As Aquinas says, charity is the greatest of all possible virtues, but clemency of all moral virtues concerned with mitigation of evil seems to be most charity-like, because it is most concerned with the good of others, on the sides both of punishment and restraint.
There is nothing in Aquinas, of course, about the vice of misericordia; misericordia is obviously a virtue for Aquinas, because it is associated with charity, the greatest Christian virtue. Both misericordia and clementia could be translated as 'mercy', but they are different, albeit analogous. Misericordia is the positive counterpart of clementia; as clementia is the greatest moral virtue concerned with mitigation of punishment, misericordia is the greatest moral virtue concerned with actively aiding our neighbor. However, being more concerned with positive good means that misericordia is the greater virtue, as the one that both resembles charity most and is more closely associated with it (since it includes within its scope external acts of charity). However, the two are enough alike that the words used to describe one are often used to describe the other, and in the same way the mercilessness that opposes misericordia is often conflated with cruelty.
In the modern period, clemency tends to not to be discussed directly; it is in the context of its major opposing vice of defect, cruelty, that it usually gets treated. This is an interesting switch, since medieval philosophy seems rarely to discuss cruelty except as a side issue in discussing clemency or mercy. Much of the modern discussion of cruelty is directly or indirectly Senecan in origin. Hume, like Seneca and Aquinas before him, takes cruelty to be associated with anger, and with a rather Senecan argument points out that it is widely despised (Treatise 3.3.3): "Where these angry passions rise up to cruelty, they form the most detested of all vices. All the pity and concern which we have for the miserable sufferers by this vice, turns against the person guilty of it, and produces a stronger hatred than we are sensible of on any other occasion." As in Seneca, cruelty is mostly associated in Hume with government action. This is not uncommon. Further, 'cruelty' has become a catch-all term; it would be difficult to make Aquinas's distinction between mercilessness and cruelty, for instance, in purely modern terms, and cruelty is often not treated as a vice, but more as an ambience created by a system.
One of the more interesting discussions that could be related to clemency is found in Bishop Butler's Fifteen Sermons; Sermons VIII and IX are devoted to resentment and forgiveness of injury, which certainly brings us very close to the matters with which clemency is concerned. Resentment or indignation, according to Butler, is the natural passion that, when sudden and hasty, we call anger, and, when deliberate and settled, we call by a variety of other names (most are associated with vice, but, as Butler notes, the passion is not itself a vice). God has given us this passion in its hasty form so that we "might be better qualified to prevent, and likewise (or perhaps chiefly) to resist and defeat sudden force, violence, and opposition, considered merely as such, and without regard to the fault or demerit of him who is the author of them." Deliberate resentment, however, is for resisting and defeating wickedness, cruelty, and the like: " It is to be considered as a weapon put into our hands by nature, against injury, injustice and cruelty." We all recognize the good that comes to society because of resentment, because people are kept in line by fear of the resentment of others.
However, we need to distinguish resentment properly deployed as a weapon against injustice from vengefulness. Resentment is medicinal, but it is a painful remedy, hard on society, and therefore should not be used unless we carefully proportion it to the case to produce greater good. In particular, it must be constrained by general benevolence, and this general benevolence must be taken so far as to include love of our enemies, who are people who have injured us. This admits of a very Stoic reading so far, but Butler, as a Christian moral philosopher, also wishes to emphasize the importance of compassion:
Though injury, injustice and oppression, the baseness of ingratitude, are the natural objects of indignation, or, if you please, of resentment, as before explained; yet they are likewise the objects of compassion, as they are their own punishment, and without repentance will for ever be so. No one ever did a designed injury to another, but at the same time he did a much greater to himself. If therefore we would consider things justly, such a one is, according to the natural course of affections, an object of compassion, as well as of displeasure: and to be affected really in this manner, I say really, in opposition to show and pretence, argues the true greatness of mind. We have an example of forgiveness in this way in its utmost perfection, and which indeed includes in it all that is good, in that prayer of our blessed Saviour on the cross: "Father, forgive them; for they know not what they do!"