Objection 1. It would seem that one virtue cannot be greater or less than another. For it is written (Apocalypse 21:16) that the sides of the city of Jerusalem are equal; and a gloss says that the sides denote the virtues. Therefore all virtues are equal; and consequently one cannot be greater than another.
Objection 2. Further, a thing that, by its nature, consists in a maximum, cannot be more or less. Now the nature of virtue consists in a maximum, for virtue is "the limit of power," as the Philosopher states (De Coelo i, text. 116); and Augustine says (De Lib. Arb. ii, 19) that "virtues are very great boons, and no one can use them to evil purpose." Therefore it seems that one virtue cannot be greater or less than another.
Objection 3. Further, the quantity of an effect is measured by the power of the agent. But perfect, viz. infused virtues, are from God Whose power is uniform and infinite. Therefore it seems that one virtue cannot be greater than another.
On the contrary, Wherever there can be increase and greater abundance, there can be inequality. Now virtues admit of greater abundance and increase: for it is written (Matthew 5:20): "Unless your justice abound more than that of the Scribes and Pharisees, you shall not enter into the kingdom of heaven": and (Proverbs 15:5): "In abundant justice there is the greatest strength [virtus]." Therefore it seems that a virtue can be greater or less than another.
I answer that, When it is asked whether one virtue can be greater than another, the question can be taken in two senses. First, as applying to virtues of different species. In this sense it is clear that one virtue is greater than another; since a cause is always more excellent than its effect; and among effects, those nearest to the cause are the most excellent. Now it is clear from what has been said (18, 5; 61, 2) that the cause and root of human good is the reason. Hence prudence which perfects the reason, surpasses in goodness the other moral virtues which perfect the appetitive power, in so far as it partakes of reason. And among these, one is better than another, according as it approaches nearer to the reason. Consequently justice, which is in the will, excels the remaining moral virtues; and fortitude, which is in the irascible part, stands before temperance, which is in the concupiscible, which has a smaller share of reason, as stated in Ethic. vii, 6.
The question can be taken in another way, as referring to virtues of the same species. In this way, according to what was said above (Question 52, Article 1), when we were treating of the intensity of habits, virtue may be said to be greater or less in two ways: first, in itself; secondly with regard to the subject that partakes of it. If we consider it in itself, we shall call it greater or little, according to the things to which it extends. Now whosoever has a virtue, e.g. temperance, has it in respect of whatever temperance extends to. But this does not apply to science and art: for every grammarian does not know everything relating to grammar. And in this sense the Stoics said rightly, as Simplicius states in his Commentary on the Predicaments, that virtue cannot be more or less, as science and art can; because the nature of virtue consists in a maximum.
If, however, we consider virtue on the part of the subject, it may then be greater or less, either in relation to different times, or in different men. Because one man is better disposed than another to attain to the mean of virtue which is defined by right reason; and this, on account of either greater habituation, or a better natural disposition, or a more discerning judgment of reason, or again a greater gift of grace, which is given to each one "according to the measure of the giving of Christ," as stated in Ephesians 4:9. And here the Stoics erred, for they held that no man should be deemed virtuous, unless he were, in the highest degree, disposed to virtue. Because the nature of virtue does not require that man should reach the mean of right reason as though it were an indivisible point, as the Stoics thought; but it is enough that he should approach the mean, as stated in Ethic. ii, 6. Moreover, one same indivisible mark is reached more nearly and more readily by one than by another: as may be seen when several arches aim at a fixed target.
Reply to Objection 1. This equality is not one of absolute quantity, but of proportion: because all virtues grow in a man proportionately, as we shall see further on (2).
Reply to Objection 2. This "limit" which belongs to virtue, can have the character of something "more" or "less" good, in the ways explained above: since, as stated, it is not an indivisible limit.
Reply to Objection 3. God does not work by necessity of nature, but according to the order of His wisdom, whereby He bestows on men various measures of virtue, according to Ephesians 4:7: "To every one of you [Vulgate: 'us'] is given grace according to the measure of the giving of Christ."
Source: Summa Theologica (New Advent)