Objection 1. It would seem that moral virtue cannot be with passion. For the Philosopher says (Topic. iv) that "a gentle man is one who is not passionate; but a patient man is one who is passionate but does not give way." The same applies to all the moral virtues. Therefore all moral virtues are without passion.
Objection 2. Further, virtue is a right affection of the soul, as health is to the body, as stated Phys. vii, text. 17: wherefore "virtue is a kind of health of the soul," as Cicero says (Quaest. Tusc. iv). But the soul's passions are "the soul's diseases," as he says in the same book. Now health is incompatible with disease. Therefore neither is passion compatible with virtue.
Objection 3. Further, moral virtue requires perfect use of reason even in particular matters. But the passions are an obstacle to this: for the Philosopher says (Ethic. vi, 5) that "pleasures destroy the judgment of prudence": and Sallust says (Catilin.) that "when they," i.e. the soul's passions, "interfere, it is not easy for the mind to grasp the truth." Therefore passion is incompatible with moral virtue.
On the contrary, Augustine says (De Civ. Dei xiv, 6): "If the will is perverse, these movements," viz. the passions, "are perverse also: but if it is upright, they are not only blameless, but even praiseworthy." But nothing praiseworthy is incompatible with moral virtue. Therefore moral virtue does not exclude the passions, but is consistent with them.
I answer that, The Stoics and Peripatetics disagreed on this point, as Augustine relates (De Civ. Dei ix, 4). For the Stoics held that the soul's passions cannot be in a wise or virtuous man: whereas the Peripatetics, who were founded by Aristotle, as Augustine says (De Civ. Dei ix, 4), maintained that the passions are compatible with moral virtue, if they be reduced to the mean.
This difference, as Augustine observes (De Civ. Dei ix, 4), was one of words rather than of opinions. Because the Stoics, through not discriminating between the intellective appetite, i.e. the will, and the sensitive appetite, which is divided into irascible and concupiscible, did not, as the Peripatetics did, distinguish the passions from the other affections of the human soul, in the point of their being movements of the sensitive appetite, whereas the other emotions of the soul, which are not passions, are movements of the intellective appetite or will; but only in the point of the passions being, as they maintained, any emotions in disaccord with reason. These emotions could not be in a wise or virtuous man if they arose deliberately: while it would be possible for them to be in a wise man, if they arose suddenly: because, in the words of Aulus Gellius [Noct. Attic. xix, 1, quoted by Augustine (De Civ. Dei ix, 4), "it is not in our power to call up the visions of the soul, known as its fancies; and when they arise from awesome things, they must needs disturb the mind of a wise man, so that he is slightly startled by fear, or depressed with sorrow," in so far as "these passions forestall the use of reason without his approving of such things or consenting thereto."
Accordingly, if the passions be taken for inordinate emotions, they cannot be in a virtuous man, so that he consent to them deliberately; as the Stoics maintained. But if the passions be taken for any movements of the sensitive appetite, they can be in a virtuous man, in so far as they are subordinate to reason. Hence Aristotle says (Ethic. ii, 3) that "some describe virtue as being a kind of freedom from passion and disturbance; this is incorrect, because the assertion should be qualified": they should have said virtue is freedom from those passions "that are not as they should be as to manner and time."
Reply to Objection 1. The Philosopher quotes this, as well as many other examples in his books on Logic, in order to illustrate, not his own mind, but that of others. It was the opinion of the Stoics that the passions of the soul were incompatible with virtue: and the Philosopher rejects this opinion (Ethic. ii, 3), when he says that virtue is not freedom from passion. It may be said, however, that when he says "a gentle man is not passionate," we are to understand this of inordinate passion.
Reply to Objection 2. This and all similar arguments which Tully brings forward in De Tusc. Quaest. iv take the passions in the execution of reason's command.
Reply to Objection 3. When a passion forestalls the judgment of reason, so as to prevail on the mind to give its consent, it hinders counsel and the judgment of reason. But when it follows that judgment, as through being commanded by reason, it helps towards the execution of reason's command.
Source: Summa Theologica (New Advent)