Objection 1. It seems that magnanimity is not a special virtue. For no special virtue is operative in every virtue. But the Philosopher states (Ethic. iv, 3) that "whatever is great in each virtue belongs to the magnanimous." Therefore magnanimity is not a special virtue.
Objection 2. Further, the acts of different virtues are not ascribed to any special virtue. But the acts of different virtues are ascribed to the magnanimous man. For it is stated in Ethic. iv, 3 that "it belongs to the magnanimous not to avoid reproof" (which is an act of prudence), "nor to act unjustly" (which is an act of justice), "that he is ready to do favors" (which is an act of charity), "that he gives his services readily" (which is an act of liberality), that "he is truthful" (which is an act of truthfulness), and that "he is not given to complaining" (which is an act of patience). Therefore magnanimity is not a special virtue.
Objection 3. Further, every virtue is a special ornament of the soul, according to the saying of Isaiah 61:10, "He hath clothed me with the garments of salvation," and afterwards he adds, "and as a bride adorned with her jewels." But magnanimity is the ornament of all the virtues, as stated in Ethic. iv. Therefore magnanimity is a general virtue.
On the contrary, The Philosopher (Ethic. ii, 7) distinguishes it from the other virtues.
I answer that, As stated above (Question 123, Article 2), it belongs to a special virtue to establish the mode of reason in a determinate matter. Now magnanimity establishes the mode of reason in a determinate matter, namely honors, as stated above (1 and 2): and honor, considered in itself, is a special good, and accordingly magnanimity considered in itself is a special virtue.
Since, however, honor is the reward of every virtue, as stated above (103, 1, ad 2), it follows that by reason of its matter it regards all the virtues.
Reply to Objection 1. Magnanimity is not about any kind of honor, but great honor. Now, as honor is due to virtue, so great honor is due to a great deed of virtue. Hence it is that the magnanimous is intent on doing great deeds in every virtue, in so far, to wit, as he tends to what is worthy of great honors.
Reply to Objection 2. Since the magnanimous tends to great things, it follows that he tends chiefly to things that involve a certain excellence, and shuns those that imply defect. Now it savors of excellence that a man is beneficent, generous and grateful. Wherefore he shows himself ready to perform actions of this kind, but not as acts of the other virtues. on the other hand, it is a proof of defect, that a man thinks so much of certain external goods or evils, that for their sake he abandons and gives up justice or any virtue whatever. Again, all concealment of the truth indicates a defect, since it seems to be the outcome of fear. Also that a man be given to complaining denotes a defect, because by so doing the mind seems to give way to external evils. Wherefore these and like things the magnanimous man avoids under a special aspect, inasmuch as they are contrary to his excellence or greatness.
Reply to Objection 3. Every virtue derives from its species a certain luster or adornment which is proper to each virtue: but further adornment results from the very greatness of a virtuous deed, through magnanimity which makes all virtues greater as stated in Ethic. iv, 3.
Source: Summa Theologica (New Advent)