Objection 1. It would seem that humility concerns, not the appetite but the judgment of reason. Because humility is opposed to pride. Now pride concerns things pertaining to knowledge: for Gregory says (Moral. xxxiv, 22) that "pride, when it extends outwardly to the body, is first of all shown in the eyes": wherefore it is written (Psalm 130:1), "Lord, my heart is not exalted, nor are my eyes lofty." Now eyes are the chief aids to knowledge. Therefore it would seem that humility is chiefly concerned with knowledge, whereby one thinks little of oneself.
Objection 2. Further, Augustine says (De Virginit. xxxi) that "almost the whole of Christian teaching is humility." Consequently nothing contained in Christian teaching is incompatible with humility. Now Christian teaching admonishes us to seek the better things, according to 1 Corinthians 12:31, "Be zealous for the better gifts." Therefore it belongs to humility to restrain not the desire of difficult things but the estimate thereof.
Objection 3. Further, it belongs to the same virtue both to restrain excessive movement, and to strengthen the soul against excessive withdrawal: thus fortitude both curbs daring and fortifies the soul against fear. Now it is magnanimity that strengthens the soul against the difficulties that occur in the pursuit of great things. Therefore if humility were to curb the desire of great things, it would follow that humility is not a distinct virtue from magnanimity, which is evidently false. Therefore humility is concerned, not with the desire but with the estimate of great things.
Objection 4. Further, Andronicus [De Affectibus] assigns humility to outward show; for he says that humility is "the habit of avoiding excessive expenditure and parade." Therefore it is not concerned with the movement of the appetite.
On the contrary, Augustine says (De Poenit. [Serm. cccli]) that "the humble man is one who chooses to be an abject in the house of the Lord, rather than to dwell in the tents of sinners." But choice concerns the appetite. Therefore humility has to do with the appetite rather than with the estimative power.
I answer that, As stated above (Article 1), it belongs properly to humility, that a man restrain himself from being borne towards that which is above him. For this purpose he must know his disproportion to that which surpasses his capacity. Hence knowledge of one's own deficiency belongs to humility, as a rule guiding the appetite. Nevertheless humility is essentially in the appetite itself; and consequently it must be said that humility, properly speaking, moderates the movement of the appetite.
Reply to Objection 1. Lofty eyes are a sign of pride, inasmuch as it excludes respect and fear: for fearing and respectful persons are especially wont to lower the eyes, as though not daring to compare themselves with others. But it does not follow from this that humility is essentially concerned with knowledge.
Reply to Objection 2. It is contrary to humility to aim at greater things through confiding in one's own powers: but to aim at greater things through confidence in God's help, is not contrary to humility; especially since the more one subjects oneself to God, the more is one exalted in God's sight. Hence Augustine says (De Virginit. xxxi): "It is one thing to raise oneself to God, and another to raise oneself up against God. He that abases himself before Him, him He raiseth up; he that raises himself up against Him, him He casteth down."
Reply to Objection 3. In fortitude there is the same reason for restraining daring and for strengthening the soul against fear: since the reason in both cases is that man should set the good of reason before dangers of death. But the reason for restraining presumptuous hope which pertains to humility is not the same as the reason for strengthening the soul against despair. Because the reason for strengthening the soul against despair is the acquisition of one's proper good lest man, by despair, render himself unworthy of a good which was competent to him; while the chief reason for suppressing presumptuous hope is based on divine reverence, which shows that man ought not to ascribe to himself more than is competent to him according to the position in which God has placed him. Wherefore humility would seem to denote in the first place man's subjection to God; and for this reason Augustine (De Serm. Dom. in Monte i, 4) ascribes humility, which he understands by poverty of spirit, to the gift of fear whereby man reveres God. Hence it follows that the relation of fortitude to daring differs from that of humility to hope. Because fortitude uses daring more than it suppresses it: so that excess of daring is more like fortitude than lack of daring is. On the other hand, humility suppresses hope or confidence in self more than it uses it; wherefore excessive self-confidence is more opposed to humility than lack of confidence is.
Reply to Objection 4. Excess in outward expenditure and parade is wont to be done with a view of boasting, which is suppressed by humility. Accordingly humility has to do, in a secondary way, with externals, as signs of the inward movement of the appetite.
Source: Summa Theologica (New Advent)