HKB
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 Obj. 1: Intellectual virtues also have their measure, as stated, and they observe the mean according as they conform to that measure.
Reply Obj. 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 Obj. 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 Hermen[e]ias. 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.
FOURTH ARTICLE [I-II, Q. 64, Art. 4]
Whether the Theological Virtues Observe the Mean?
Objection 1: It would seem that theological virtue observes the mean. For the good of other virtues consists in their observing the mean. Now the theological virtues surpass the others in goodness. Therefore much more does theological virtue observe the mean.
Obj. 2: Further, the mean of moral virtue depends on the appetite being ruled by reason; while the mean of intellectual virtue consists in the intellect being measured by things. Now theological virtue perfects both intellect and appetite, as stated above (Q. 62, A. 3). Therefore theological virtue also observes the mean.
Obj. 3: Further, hope, which is a theological virtue, is a mean between despair and presumption. Likewise faith holds a middle course between contrary heresies, as Boethius states (De Duab. Natur. vii): thus, by confessing one Person and two natures in Christ, we observe the mean between the heresy of Nestorius, who maintained the existence of two persons and two natures, and the heresy of Eutyches, who held to one person and one nature. Therefore theological virtue observes the mean.