Objection 1. It would seem that moral virtue does not observe the mean. For the nature of a mean is incompatible with that which is extreme. Now the nature of virtue is to be something extreme; for it is stated in De Coelo i that "virtue is the limit of power." Therefore moral virtue does not observe the mean.
Objection 2. Further, the maximum is not a mean. Now some moral virtues tend to a maximum: for instance, magnanimity to very great honors, and magnificence to very large expenditure, as stated in Ethic. iv, 2,3. Therefore not every moral virtue observes the mean.
Objection 3. Further, if it is essential to a moral virtue to observe the mean, it follows that a moral virtue is not perfected, but the contrary corrupted, through tending to something extreme. Now some moral virtues are perfected by tending to something extreme; thus virginity, which abstains from all sexual pleasure, observes the extreme, and is the most perfect chastity: and to give all to the poor is the most perfect mercy or liberality. Therefore it seems that it is not essential to moral virtue that it should observe the mean.
On the contrary, The Philosopher says (Ethic. ii, 6) that "moral virtue is a habit of choosing the mean."
I answer that, As already explained (55, 3), the nature of virtue is that it should direct man to good. Now moral virtue is properly a perfection of the appetitive part of the soul in regard to some determinate matter: and the measure or rule of the appetitive movement in respect of appetible objects is the reason. But the good of that which is measured or ruled consists in its conformity with its rule: thus the good things made by art is that they follow the rule of art. Consequently, in things of this sort, evil consists in discordance from their rule or measure. Now this may happen either by their exceeding the measure or by their falling short of it; as is clearly the case in all things ruled or measured. Hence it is evident that the good of moral virtue consists in conformity with the rule of reason. Now it is clear that between excess and deficiency the mean is equality or conformity. Therefore it is evident that moral virtue observes the mean.
Reply to Objection 1. Moral virtue derives goodness from the rule of reason, while its matter consists in passions or operations. If therefore we compare moral virtue to reason, then, if we look at that which is has of reason, it holds the position of one extreme, viz. conformity; while excess and defect take the position of the other extreme, viz. deformity. But if we consider moral virtue in respect of its matter, then it holds the position of mean, in so far as it makes the passion conform to the rule of reason. Hence the Philosopher says (Ethic. ii, 6) that "virtue, as to its essence, is a mean state," in so far as the rule of virtue is imposed on its proper matter: "but it is an extreme in reference to the 'best' and the 'excellent,'" viz. as to its conformity with reason.
Reply to Objection 2. In actions and passions the mean and the extremes depend on various circumstances: hence nothing hinders something from being extreme in a particular virtue as to one circumstance, while the same thing is a mean in respect of other circumstances, through being in conformity with reason. This is the case with magnanimity and magnificence. For if we look at the absolute quantity of the respective objects of these virtues, we shall call it an extreme and a maximum: but if we consider the quantity in relation to other circumstances, then it has the character of a mean: since these virtues tend to this maximum in accordance with the rule of reason, i.e. "where" it is right, "when" it is right, and for an "end" that is right. There will be excess, if one tends to this maximum "when" it is not right, or "where" it is not right, or for an undue "end"; and there will be deficiency if one fails to tend thereto "where" one ought, and "when" one aught. This agrees with the saying of the Philosopher (Ethic. iv, 3) that the "magnanimous man observes the extreme in quantity, but the mean in the right mode of his action."
Reply to Objection 3. The same is to be said of virginity and poverty as of magnanimity. For virginity abstains from all sexual matters, and poverty from all wealth, for a right end, and in a right manner, i.e. according to God's word, and for the sake of eternal life. But if this be done in an undue manner, i.e. out of unlawful superstition, or again for vainglory, it will be in excess. And if it be not done when it ought to be done, or as it ought to be done, it is a vice by deficiency: for instance, in those who break their vows of virginity or poverty.
Source: Summa Theologica (New Advent)