Objection 1. It would seem that it does not belong to prudence to find the mean in moral virtues. For the achievement of the mean is the end of moral virtues. But prudence does not appoint the end to moral virtues, as shown above (Article 6). Therefore it does not find the mean in them.
Objection 2. Further, that which of itself has being, would seem to have no cause, but its very being is its cause, since a thing is said to have being by reason of its cause. Now "to follow the mean" belongs to moral virtue by reason of itself, as part of its definition, as shown above (5, Objection 1). Therefore prudence does not cause the mean in moral virtues.
Objection 3. Further, prudence works after the manner of reason. But moral virtue tends to the mean after the manner of nature, because, as Tully states (De Invent. Rhet. ii, 53), "virtue is a habit like a second nature in accord with reason." Therefore prudence does not appoint the mean to moral virtues.
On the contrary, In the foregoing definition of moral virtue (5, Objection 1) it is stated that it "follows a mean appointed by reason... even as a wise man decides."
I answer that, The proper end of each moral virtue consists precisely in conformity with right reason. For temperance intends that man should not stray from reason for the sake of his concupiscences; fortitude, that he should not stray from the right judgment of reason through fear or daring. Moreover this end is appointed to man according to natural reason, since natural reason dictates to each one that he should act according to reason.
But it belongs to the ruling of prudence to decide in what manner and by what means man shall obtain the mean of reason in his deeds. For though the attainment of the mean is the end of a moral virtue, yet this mean is found by the right disposition of these things that are directed to the end.
This suffices for the Reply to the First Objection.
Reply to Objection 2. Just as a natural agent makes form to be in matter, yet does not make that which is essential to the form to belong to it, so too, prudence appoints the mean in passions and operations, and yet does not make the searching of the mean to belong to virtue.
Reply to Objection 3. Moral virtue after the manner of nature intends to attain the mean. Since, however, the mean as such is not found in all matters after the same manner, it follows that the inclination of nature which ever works in the same manner, does not suffice for this purpose, and so the ruling of prudence is required.
Source: Summa Theologica (New Advent)