Objection 1. It would seem that neither clemency nor meekness is a virtue. For no virtue is opposed to another virtue. Yet both of these are apparently opposed to severity, which is a virtue. Therefore neither clemency nor meekness is a virtue.
Objection 2. Further, "Virtue is destroyed by excess and defect" [Ethic. ii, 2. But both clemency and meekness consist in a certain decrease; for clemency decreases punishment, and meekness decreases anger. Therefore neither clemency nor meekness is a virtue.
Objection 3. Further, meekness or mildness is included (Matthew 5:4) among the beatitudes, and (Galatians 5:23) among the fruits. Now the virtues differ from the beatitudes and fruits. Therefore they are not comprised under virtue.
On the contrary, Seneca says (De Clementia ii, 5): "Every good man is conspicuous for his clemency and meekness." Now it is virtue properly that belongs to a good man, since "virtue it is that makes its possessor good, and renders his works good also" (Ethic. ii, 6). Therefore clemency and meekness are virtues.
I answer that, The nature of moral virtue consists in the subjection of appetite to reason, as the Philosopher declares (Ethic. i, 13). Now this is verified both in clemency and in meekness. For clemency, in mitigating punishment, "is guided by reason," according to Seneca (De Clementia ii, 5), and meekness, likewise, moderates anger according to right reason, as stated in Ethic. iv, 5. Wherefore it is manifest that both clemency and meekness are virtues.
Reply to Objection 1. Meekness is not directly opposed to severity; for meekness is about anger. On the other hand, severity regards the external infliction of punishment, so that accordingly it would seem rather to be opposed to clemency, which also regards external punishing, as stated above (Article 1). Yet they are not really opposed to one another, since they are both according to right reason. For severity is inflexible in the infliction of punishment when right reason requires it; while clemency mitigates punishment also according to right reason, when and where this is requisite. Wherefore they are not opposed to one another as they are not about the same thing.
Reply to Objection 2. According to the Philosopher (Ethic. iv, 5), "the habit that observes the mean in anger is unnamed; so that the virtue is denominated from the diminution of anger, and is designated by the name of meekness." For the virtue is more akin to diminution than to excess, because it is more natural to man to desire vengeance for injuries done to him, than to be lacking in that desire, since "scarcely anyone belittles an injury done to himself," as Sallust observes [Cf. 120]. As to clemency, it mitigates punishment, not in respect of that which is according to right reason, but as regards that which is according to common law, which is the object of legal justice: yet on account of some particular consideration, it mitigates the punishment, deciding, as it were, that a man is not to be punished any further. Hence Seneca says (De Clementia ii, 1): "Clemency grants this, in the first place, that those whom she sets free are declared immune from all further punishment; and remission of punishment due amounts to a pardon." Wherefore it is clear that clemency is related to severity as equity [the Greek 'epieikeia' [Cf. 120]] to legal justice, whereof severity is a part, as regards the infliction of punishment in accordance with the law. Yet clemency differs from equity, as we shall state further on (3, ad 1).
Reply to Objection 3. The beatitudes are acts of virtue: while the fruits are delights in virtuous acts. Wherefore nothing hinders meekness being reckoned both virtue, and beatitude and fruit.
Source: Summa Theologica (New Advent)