Objection 1. It seems that goods of fortune do not conduce to magnanimity. For according to Seneca (De Ira i: De vita beata xvi): "virtue suffices for itself." Now magnanimity takes every virtue great, as stated above (4, ad 3). Therefore goods of fortune do not conduce to magnanimity.
Objection 2. Further, no virtuous man despises what is helpful to him. But the magnanimous man despises whatever pertains to goods of fortune: for Tully says (De Offic. i) under the heading: "Magnanimity consists of two things," that "a great soul is commended for despising external things." Therefore a magnanimous man is not helped by goods of fortune.
Objection 3. Further, Tully adds (De Offic. i) that "it belongs to a great soul so to bear what seems troublesome, as nowise to depart from his natural estate, or from the dignity of a wise man." And Aristotle says (Ethic. iv, 3) that "a magnanimous man does not grieve at misfortune." Now troubles and misfortunes are opposed to goods of fortune, for every one grieves at the loss of what is helpful to him. Therefore external goods of fortune do not conduce to magnanimity.
On the contrary, The Philosopher says (Ethic. iv, 3) that "good fortune seems to conduce to magnanimity."
I answer that, As stated above (Article 1), magnanimity regards two things: honor as its matter, and the accomplishment of something great as its end. Now goods of fortune conduce to both these things. For since honor is conferred on the virtuous, not only by the wise, but also by the multitude who hold these goods of fortune in the highest esteem, the result is that they show greater honor to those who possess goods of fortune. Likewise goods of fortune are useful organs or instruments of virtuous deeds: since we can easily accomplish things by means of riches, power and friends. Hence it is evident that goods of fortune conduce to magnanimity.
Reply to Objection 1. Virtue is said to be sufficient for itself, because it can be without even these external goods; yet it needs them in order to act more expeditiously.
Reply to Objection 2. The magnanimous man despises external goods, inasmuch as he does not think them so great as to be bound to do anything unbecoming for their sake. Yet he does not despise them, but that he esteems them useful for the accomplishment of virtuous deeds.
Reply to Objection 3. If a man does not think much of a thing, he is neither very joyful at obtaining it, nor very grieved at losing it. Wherefore, since the magnanimous man does not think much of external goods, that is goods of fortune, he is neither much uplifted by them if he has them, nor much cast down by their loss.
Source: Summa Theologica (New Advent)