Objection 1. It would seem that the subject of pride is not the irascible faculty. For Gregory says (Moral. xxiii, 17): "A swollen mind is an obstacle to truth, for the swelling shuts out the light." Now the knowledge of truth pertains, not to the irascible but to the rational faculty. Therefore pride is not in the irascible.
Objection 2. Further, Gregory says (Moral. xxiv, 8) that "the proud observe other people's conduct not so as to set themselves beneath them with humility, but so as to set themselves above them with pride": wherefore it would seem that pride originates in undue observation. Now observation pertains not to the irascible but to the rational faculty.
Objection 3. Further. pride seeks pre-eminence not only in sensible things, but also in spiritual and intelligible things: while it consists essentially in the contempt of God, according to Sirach 10:14, "The beginning of the pride of man is to fall off from God." Now the irascible, since it is a part of the sensitive appetite, cannot extend to God and things intelligible. Therefore pride cannot be in the irascible.
Objection 4. Further, as stated in Prosper's Liber Sententiarum, sent. 294, "Pride is love of one's own excellence." But love is not in the irascible, but in the concupiscible. Therefore pride is not in the irascible.
On the contrary, Gregory (Moral. ii, 49) opposes pride to the gift of fear. Now fear belongs to the irascible. Therefore pride is in the irascible.
I answer that, The subject of any virtue or vice is to be ascertained from its proper object: for the object of a habit or act cannot be other than the object of the power, which is the subject of both. Now the proper object of pride is something difficult, for pride is the desire of one's own excellence, as stated above (1 and 2). Wherefore pride must needs pertain in some way to the irascible faculty. Now the irascible may be taken in two ways. First in a strict sense, and thus it is a part of the sensitive appetite, even as anger, strictly speaking, is a passion of the sensitive appetite. Secondly, the irascible may be taken in a broader sense, so as to belong also to the intellective appetite, to which also anger is sometimes ascribed. It is thus that we attribute anger to God and the angels, not as a passion, but as denoting the sentence of justice pronouncing judgment. Nevertheless the irascible understood in this broad sense is not distinct from the concupiscible power, as stated above in I, 59, 4; I-II, 82, 5, ad 1 and 2.
Consequently if the difficult thing which is the object of pride, were merely some sensible object, whereto the sensitive appetite might tend, pride would have to be in the irascible which is part of the sensitive appetite. But since the difficult thing which pride has in view is common both to sensible and to spiritual things, we must needs say that the subject of pride is the irascible not only strictly so called, as a part of the sensitive appetite, but also in its wider acceptation, as applicable to the intellective appetite. Wherefore pride is ascribed also to the demons.
Reply to Objection 1. Knowledge of truth is twofold. One is purely speculative, and pride hinders this indirectly by removing its cause. For the proud man subjects not his intellect to God, that he may receive the knowledge of truth from Him, according to Matthew 11:25, "Thou hast hid these things from the wise and the prudent," i.e. from the proud, who are wise and prudent in their own eyes, "and hast revealed them to little ones," i.e. to the humble.
Nor does he deign to learn anything from man, whereas it is written (Sirach 6:34): "If thou wilt incline thy ear, thou shalt receive instruction." The other knowledge of truth is affective, and this is directly hindered by pride, because the proud, through delighting in their own excellence, disdain the excellence of truth; thus Gregory says (Moral. xxiii, 17) that "the proud, although certain hidden truths be conveyed to their understanding, cannot realize their sweetness: and if they know of them they cannot relish them." Hence it is written (Proverbs 11:2): "Where humility is there also is wisdom."
Reply to Objection 2. As stated above (161, 2, 6), humility observes the rule of right reason whereby a man has true self-esteem. Now pride does not observe this rule of right reason, for he esteems himself greater than he is: and this is the outcome of an inordinate desire for his own excellence, since a man is ready to believe what he desires very much, the result being that his appetite is borne towards things higher than what become him. Consequently whatsoever things lead a man to inordinate self-esteem lead him to pride: and one of those is the observing of other people's failings, just as, on the other hand, in the words of Gregory (Moral. xxiii, 17), "holy men, by a like observation of other people's virtues, set others above themselves." Accordingly the conclusion is not that pride is in the rational faculty, but that one of its causes is in the reason.
Reply to Objection 3. Pride is in the irascible, not only as a part of the sensitive appetite, but also as having a more general signification, as stated above.
Reply to Objection 4. According to Augustine (De Civ. Dei xiv, 7,9), "love precedes all other emotions of the soul, and is their cause," wherefore it may be employed to denote any of the other emotions. It is in this sense that pride is said to be "love of one's own excellence," inasmuch as love makes a man presume inordinately on his superiority over others, and this belongs properly to pride.
Source: Summa Theologica (New Advent)