Objection 1. It seems that pusillanimity is not opposed to magnanimity. For the Philosopher says (Ethic., 3) that "the fainthearted man knows not himself: for he would desire the good things, of which he is worthy, if he knew himself." Now ignorance of self seems opposed to prudence. Therefore pusillanimity is opposed to prudence.
Objection 2. Further our Lord calls the servant wicked and slothful who through pusillanimity refused to make use of the money. Moreover the Philosopher says (Ethic. iv, 3) that the fainthearted seem to be slothful. Now sloth is opposed to solicitude, which is an act of prudence, as stated above (Question 47, Article 09). Therefore pusillanimity is not opposed to magnanimity.
Objection 3. Further, pusillanimity seems to proceed from inordinate fear: hence it is written (Isaiah 35:4): "Say to the fainthearted: Take courage and fear not." It also seems to proceed from inordinate anger, according to Colossians 3:21, "Fathers, provoke not your children to indignation, lest they be discouraged." Now inordinate fear is opposed to fortitude, and inordinate anger to meekness. Therefore pusillanimity is not opposed to magnanimity.
Objection 4. Further, the vice that is in opposition to a particular virtue is the more grievous according as it is more unlike that virtue. Now pusillanimity is more unlike magnanimity than presumption is. Therefore if pusillanimity is opposed to magnanimity, it follows that it is a more grievous sin than presumption: yet this is contrary to the saying of Sirach 37:3, "O wicked presumption, whence camest thou?" Therefore pusillanimity is not opposed to magnanimity.
On the contrary, Pusillanimity and magnanimity differ as greatness and littleness of soul, as their very names denote. Now great and little are opposites. Therefore pusillanimity is opposed to magnanimity.
I answer that, Pusillanimity may be considered in three ways. First, in itself; and thus it is evident that by its very nature it is opposed to magnanimity, from which it differs as great and little differ in connection with the same subject. For just as the magnanimous man tends to great things out of greatness of soul, so the pusillanimous man shrinks from great things out of littleness of soul. Secondly, it may be considered in reference to its cause, which on the part of the intellect is ignorance of one's own qualification, and on the part of the appetite is the fear of failure in what one falsely deems to exceed one's ability. Thirdly, it may be considered in reference to its effect, which is to shrink from the great things of which one is worthy. But, as stated above (132, 2, ad 3), opposition between vice and virtue depends rather on their respective species than on their cause or effect. Hence pusillanimity is directly opposed to magnanimity.
Reply to Objection 1. This argument considers pusillanimity as proceeding from a cause in the intellect. Yet it cannot be said properly that it is opposed to prudence, even in respect of its cause: because ignorance of this kind does not proceed from indiscretion but from laziness in considering one's own ability, according to Ethic. iv, 3, or in accomplishing what is within one's power.
Reply to Objection 2. This argument considers pusillanimity from the point of view of its effect.
Reply to Objection 3. This argument considers the point of view of cause. Nor is the fear that causes pusillanimity always a fear of the dangers of death: wherefore it does not follow from this standpoint that pusillanimity is opposed to fortitude. As regards anger, if we consider it under the aspect of its proper movement, whereby a man is roused to take vengeance, it does not cause pusillanimity, which disheartens the soul; on the contrary, it takes it away. If, however, we consider the causes of anger, which are injuries inflicted whereby the soul of the man who suffers them is disheartened, it conduces to pusillanimity.
Reply to Objection 4. According to its proper species pusillanimity is a graver sin than presumption, since thereby a man withdraws from good things, which is a very great evil according to Ethic. iv. Presumption, however, is stated to be "wicked" on account of pride whence it proceeds.
Source: Summa Theologica (New Advent)