Objection 1. It seems that magnanimity is not a virtue. For every moral virtue observes the mean. But magnanimity observes not the mean but the greater extreme: because the "magnanimous man deems himself worthy of the greatest things" (Ethic. iv, 3). Therefore magnanimity is not a virtue.
Objection 2. Further, he that has one virtue has them all, as stated above (I-II, 65, 1). But one may have a virtue without having magnanimity: since the Philosopher says (Ethic. iv, 3) that "whosoever is worthy of little things and deems himself worthy of them, is temperate, but he is not magnanimous." Therefore magnanimity is not a virtue.
Objection 3. Further, "Virtue is a good quality of the mind," as stated above (I-II, 55, 4). But magnanimity implies certain dispositions of the body: for the Philosopher says (Ethic. iv, 3) of "a magnanimous man that his gait is slow, his voice deep, and his utterance calm." Therefore magnanimity is not a virtue.
Objection 4. Further, no virtue is opposed to another virtue. But magnanimity is opposed to humility, since "the magnanimous deems himself worthy of great things, and despises others," according to Ethic. iv, 3. Therefore magnanimity is not a virtue.
Objection 5. Further, the properties of every virtue are praiseworthy. But magnanimity has certain properties that call for blame. For, in the first place, the magnanimous is unmindful of favors; secondly, he is remiss and slow of action; thirdly, he employs irony [Cf. 113] towards many; fourthly, he is unable to associate with others; fifthly, because he holds to the barren things rather than to those that are fruitful. Therefore magnanimity is not a virtue.
On the contrary, It is written in praise of certain men (2 Maccabees 15:18): "Nicanor hearing of the valor of Judas' companions, and the greatness of courage [animi magnitudinem] with which they fought for their country, was afraid to try the matter by the sword." Now, only deeds of virtue are worthy of praise. Therefore magnanimity which consists in greatness of courage is a virtue.
I answer that, The essence of human virtue consists in safeguarding the good of reason in human affairs, for this is man's proper good. Now among external human things honors take precedence of all others, as stated above (1; I-II, 11, 2, Objection 3). Therefore magnanimity, which observes the mode of reason in great honors, is a virtue.
Reply to Objection 1. As the Philosopher again says (Ethic. iv, 3), "the magnanimous in point of quantity goes to extremes," in so far as he tends to what is greatest, "but in the matter of becomingness, he follows the mean," because he tends to the greatest things according to reason, for "he deems himself worthy in accordance with his worth" (Ethic. iv, 3), since his aims do not surpass his deserts.
Reply to Objection 2. The mutual connection of the virtues does not apply to their acts, as though every one were competent to practice the acts of all the virtues. Wherefore the act of magnanimity is not becoming to every virtuous man, but only to great men. on the other hand, as regards the principles of virtue, namely prudence and grace, all virtues are connected together, since their habits reside together in the soul, either in act or by way of a proximate disposition thereto. Thus it is possible for one to whom the act of magnanimity is not competent, to have the habit of magnanimity, whereby he is disposed to practice that act if it were competent to him according to his state.
Reply to Objection 3. The movements of the body are differentiated according to the different apprehensions and emotions of the soul. And so it happens that to magnanimity there accrue certain fixed accidents by way of bodily movements. For quickness of movement results from a man being intent on many things which he is in a hurry to accomplish, whereas the magnanimous is intent only on great things; these are few and require great attention, wherefore they call for slow movement. Likewise shrill and rapid speaking is chiefly competent to those who are quick to quarrel about anything, and this becomes not the magnanimous who are busy only about great things. And just as these dispositions of bodily movements are competent to the magnanimous man according to the mode of his emotions, so too in those who are naturally disposed to magnanimity these conditions are found naturally.
Reply to Objection 4. There is in man something great which he possesses through the gift of God; and something defective which accrues to him through the weakness of nature. Accordingly magnanimity makes a man deem himself worthy of great things in consideration of the gifts he holds from God: thus if his soul is endowed with great virtue, magnanimity makes him tend to perfect works of virtue; and the same is to be said of the use of any other good, such as science or external fortune. On the other hand, humility makes a man think little of himself in consideration of his own deficiency, and magnanimity makes him despise others in so far as they fall away from God's gifts: since he does not think so much of others as to do anything wrong for their sake. Yet humility makes us honor others and esteem them better than ourselves, in so far as we see some of God's gifts in them. Hence it is written of the just man (Psalm 14:4): "In his sight a vile person is contemned [Douay: 'The malignant is brought to nothing, but he glorifieth,' etc.]," which indicates the contempt of magnanimity, "but he honoreth them that fear the Lord," which points to the reverential bearing of humility. It is therefore evident that magnanimity and humility are not contrary to one another, although they seem to tend in contrary directions, because they proceed according to different considerations.
Reply to Objection 5. These properties in so far as they belong to a magnanimous man call not for blame, but for very great praise. For in the first place, when it is said that the magnanimous is not mindful of those from whom he has received favors, this points to the fact that he takes no pleasure in accepting favors from others unless he repay them with yet greater favor; this belongs to the perfection of gratitude, in the act of which he wishes to excel, even as in the acts of other virtues. Again, in the second place, it is said that he is remiss and slow of action, not that he is lacking in doing what becomes him, but because he does not busy himself with all kinds of works, but only with great works, such as are becoming to him. He is also said, in the third place, to employ irony, not as opposed to truth, and so as either to say of himself vile things that are not true, or deny of himself great things that are true, but because he does not disclose all his greatness, especially to the large number of those who are beneath him, since, as also the Philosopher says (Ethic. iv, 3), "it belongs to a magnanimous man to be great towards persons of dignity and affluence, and unassuming towards the middle class." On the fourth place, it is said that he cannot associate with others: this means that he is not at home with others than his friends: because he altogether shuns flattery and hypocrisy, which belong to littleness of mind. But he associates with all, both great and little, according as he ought, as stated above (ad 1). It is also said, fifthly, that he prefers to have barren things, not indeed any, but good, i.e. virtuous; for in all things he prefers the virtuous to the useful, as being greater: since the useful is sought in order to supply a defect which is inconsistent with magnanimity.
Source: Summa Theologica (New Advent)