Objection 1. It seems that magnanimity is not a part of fortitude. For a thing is not a part of itself. But magnanimity appears to be the same as fortitude. For Seneca says (De Quat. Virtut.): "If magnanimity, which is also called fortitude, be in thy soul, thou shalt live in great assurance": and Tully says (De Offic. i): "If a man is brave we expect him to be magnanimous, truth-loving, and far removed from deception." Therefore magnanimity is not a part of fortitude.
Objection 2. Further, the Philosopher (Ethic. iv, 3) says that a magnanimous man is not philokindynos, that is, a lover of danger. But it belongs to a brave man to expose himself to danger. Therefore magnanimity has nothing in common with fortitude so as to be called a part thereof.
Objection 3. Further, magnanimity regards the great in things to be hoped for, whereas fortitude regards the great in things to be feared or dared. But good is of more import than evil. Therefore magnanimity is a more important virtue than fortitude. Therefore it is not a part thereof.
On the contrary, Macrobius (De Somn. Scip. i) and Andronicus reckon magnanimity as a part of fortitude.
I answer that, As stated above (I-II, 61, 3), a principal virtue is one to which it belongs to establish a general mode of virtue in a principal matter. Now one of the general modes of virtue is firmness of mind, because "a firm standing is necessary in every virtue," according to Ethic. ii. And this is chiefly commended in those virtues that tend to something difficult, in which it is most difficult to preserve firmness. Wherefore the more difficult it is to stand firm in some matter of difficulty, the more principal is the virtue which makes the mind firm in that matter.
Now it is more difficult to stand firm in dangers of death, wherein fortitude confirms the mind, than in hoping for or obtaining the greatest goods, wherein the mind is confirmed by magnanimity, for, as man loves his life above all things, so does he fly from dangers of death more than any others. Accordingly it is clear that magnanimity agrees with fortitude in confirming the mind about some difficult matter; but it falls short thereof, in that it confirms the mind about a matter wherein it is easier to stand firm. Hence magnanimity is reckoned a part of fortitude, because it is annexed thereto as secondary to principal.
Reply to Objection 1. As the Philosopher says (Ethic. v, 1,3), "to lack evil is looked upon as a good," wherefore not to be overcome by a grievous evil, such as the danger of death, is looked upon as though it were the obtaining of a great good, the former belonging to fortitude, and the latter to magnanimity: in this sense fortitude and magnanimity may be considered as identical. Since, however, there is a difference as regards the difficulty on the part of either of the aforesaid, it follows that properly speaking magnanimity, according to the Philosopher (Ethic. ii, 7), is a distinct virtue from fortitude.
Reply to Objection 2. A man is said to love danger when he exposes himself to all kinds of dangers, which seems to be the mark of one who thinks "many" the same as "great." This is contrary to the nature of a magnanimous man, for no one seemingly exposes himself to danger for the sake of a thing that he does not deem great. But for things that are truly great, a magnanimous man is most ready to expose himself to danger, since he does something great in the act of fortitude, even as in the acts of the other virtues. Hence the Philosopher says (Ethic. ii, 7) that the magnanimous man is not mikrokindynos, i.e. endangering himself for small things, but megalokindynos, i.e. endangering himself for great things. And Seneca says (De Quat. Virtut.): "Thou wilt be magnanimous if thou neither seekest dangers like a rash man, nor fearest them like a coward. For nothing makes the soul a coward save the consciousness of a wicked life."
Reply to Objection 3. Evil as such is to be avoided: and that one has to withstand it is accidental; in so far, to wit, as one has to suffer an evil in order to safeguard a good. But good as such is to be desired, and that one avoids it is only accidental, in so far, to wit, as it is deemed to surpass the ability of the one who desires it. Now that which is so essentially is always of more account than that which is so accidentally. Wherefore the difficult in evil things is always more opposed to firmness of mind than the difficult in good things. Hence the virtue of fortitude takes precedence of the virtue of magnanimity. For though good is simply of more import than evil, evil is of more import in this particular respect.
Source: Summa Theologica (New Advent)