Aristotle famously teaches that virtue is a mean between two extremes. For example, in Nichomachean Ethics 2.7 he claims that liberality is the mean between the extremes of prodigality and illiberality.
But this sort of position is open to misunderstanding. It might, for instance, lead one to conclude that the mean for marital virtue lies between the extremes of adultery and fidelity.
Virtue, then, is a habit or trained faculty of choice, the characteristic of which lies in moderation or observance of the mean relatively to the persons concerned, as determined by reason, i.e. by the reason by which the prudent man would determine it. And it is a moderation, firstly, inasmuch as it comes in the middle or mean between two vices, one on the side of excess, the other on the side of defect; and, secondly, inasmuch as, while these vices fall short of or exceed the due measure in feeling and in action, it finds and chooses the mean, middling, or moderate amount.
Regarded in its essence, therefore, or according to the definition of its nature, virtue is a moderation or middle state, but viewed in its relation to what is best and right it is the extreme of perfection.
But it is not all actions nor all passions that admit of moderation; there are some whose very names imply badness, as malevolence, shamelessness, envy, and, among acts, adultery, theft, murder.2 These and all other like things are blamed as being bad in themselves, and not merely in their excess or deficiency. It is impossible therefore to go right in them; they are always wrong: rightness and wrongness in such things (e.g. in adultery) does not depend upon whether it is the right person and occasion and manner, but the mere doing of any one of them is wrong.
It would be equally absurd to look for moderation or excess or deficiency in unjust cowardly or profligate conduct; for then there would be moderation in excess or deficiency, and excess in excess, and deficiency in deficiency.
The fact is that just as there can be no excess or deficiency in temperance or courage because the mean or moderate amount is, in a sense, an extreme, so in these kinds of conduct also there can be no moderation or excess or deficiency, but the acts are wrong however they be done. For, to put it generally, there cannot be moderation in excess or deficiency, nor excess or deficiency in moderation.