Objection 1. It would seem that the intellectual virtues do not observe the mean. Because moral virtue observes the mean by conforming to the rule of reason. But the intellectual virtues are in reason itself, so that they seem to have no higher rule. Therefore the intellectual virtues do not observe the mean.
Objection 2. Further, the mean of moral virtue is fixed by an intellectual virtue: for it is stated in Ethic. ii, 6, that "virtue observes the mean appointed by reason, as a prudent man would appoint it." If therefore intellectual virtue also observe the mean, this mean will have to be appointed for them by another virtue, so that there would be an indefinite series of virtues.
Objection 3. Further, a mean is, properly speaking, between contraries, as the Philosopher explains (Metaph. x, text. 22,23). But there seems to be no contrariety in the intellect; since contraries themselves, as they are in the intellect, are not in opposition to one another, but are understood together, as white and black, healthy and sick. Therefore there is no mean in the intellectual virtues.
On the contrary, Art is an intellectual virtue; and yet there is a mean in art (Ethic. ii, 6). Therefore also intellectual virtue observes the mean.
I answer that, The good of anything consists in its observing the mean, by conforming with a rule or measure in respect of which it may happen to be excessive or deficient, as stated above (Article 1). Now intellectual virtue, like moral virtue, is directed to the good, as stated above (Question 56, Article 3). Hence the good of an intellectual virtue consists in observing the mean, in so far as it is subject to a measure. Now the good of intellectual virtue is the true; in the case of contemplative virtue, it is the true taken absolutely (Ethic. vi, 2); in the case of practical virtue, it is the true in conformity with a right appetite.
Now truth apprehended by our intellect, if we consider it absolutely, is measured by things; since things are the measure of our intellect, as stated in Metaph. x, text. 5; because there is truth in what we think or say, according as the thing is so or not. Accordingly the good of speculative intellectual virtue consists in a certain mean, by way of conformity with things themselves, in so far as the intellect expresses them as being what they are, or as not being what they are not: and it is in this that the nature of truth consists. There will be excess if something false is affirmed, as though something were, which in reality it is not: and there will be deficiency if something is falsely denied, and declared not to be, whereas in reality it is.
The truth of practical intellectual virtue, if we consider it in relation to things, is by way of that which is measured; so that both in practical and in speculative intellectual virtues, the mean consists in conformity with things. But if we consider it in relation to the appetite, it has the character of a rule and measure. Consequently the rectitude of reason is the mean of moral virtue, and also the mean of prudence--of prudence as ruling and measuring, of moral virtue, as ruled and measured by that mean. In like manner the difference between excess and deficiency is to be applied in both cases.
Reply to Objection 1. Intellectual virtues also have their measure, as stated, and they observe the mean according as they conform to that measure.
Reply to Objection 2. There is no need for an indefinite series of virtues: because the measure and rule of intellectual virtue is not another kind of virtue, but things themselves.
Reply to Objection 3. The things themselves that are contrary have no contrariety in the mind, because one is the reason for knowing the other: nevertheless there is in the intellect contrariety of affirmation and negation, which are contraries, as stated at the end of Peri Hermenias. For though "to be" and "not to be" are not in contrary, but in contradictory opposition to one another, so long as we consider their signification in things themselves, for on the one hand we have "being" and on the other we have simply "non-being"; yet if we refer them to the act of the mind, there is something positive in both cases. Hence "to be" and "not to be" are contradictory: but the opinion stating that "good is good" is contrary to the opinion stating that "good is not good": and between two such contraries intellectual virtue observes the mean.
Source: Summa Theologica (New Advent)