Thomas Aquinas

“ 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. ”

“ 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. ”

“ Hope observes the mean between presumption and despair, in relation to us, in so far, to wit, as a man is said to be presumptuous, through hoping to receive from God a good in excess of his condition; or to despair through failing to hope for that which according to his condition he might hope for. But there can be no excess of hope in comparison with God, Whose goodness is infinite. In like manner faith holds a middle course between contrary heresies, not by comparison with its object, which is God, in Whom we cannot believe too much ”

“ 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" ”

“ 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. ”