To differentiate virtue from the other members of its class, the following proposition is relevant — that every virtue influences or affects that of which it is the virtue in two ways; (a) it produces a good state in it, (b) it enables it to perform its function well (e.g., the virtue of the eye makes both the eye and its function good, for good sight is due to excellence of the eye). In accordance with this proposition it can be said that virtue in man is whatever characteristic makes him a good man and causes him to perform his function well.
Any continuous activity (including feeling and action, the raw materials of virtue) is divisible into parts. These may include a larger part, a smaller part, and the half or equal part, which can be defined as the mean between too much and too little. In things which do not vary there is an objective mean which is always the same (e.g., the mean between two points ten miles apart is always five miles).
The mean is relative in such things as the feelings and actions of men. This is because there are differences between people in regard to most characteristics and attributes (e.g., ten pounds of food may be too much for a man and two pounds not enough, but this does not necessarily imply that six pounds is the right amount for him or for all men).
All arts and crafts aim at this relative mean (e.g., nothing can be added to or taken from a perfect work of art without destroying it). In the same way moral virtue aims at the relative mean in feeling and action. Moral virtue can be defined as a disposition to choose the mean relative to oneself, as determined by a rational principle (i.e., by the rational principle that would be applied by a man with practical wisdom and common sense).
It is possible to experience too much or too little of any emotion, and in either case the emotion is not experienced properly. The mark of virtue is to experience an emotion at the right time, toward the right objects or people, for the right reason, and in the right manner; in other words, in accordance with the mean. This principle applies to the evaluation of all human actions.
As already shown, virtue is concerned with emotions and actions. In judging emotions and actions, we criticize excess and deficiency and praise the mean, which is construed by most people to constitute success. Both praise and success are signs of virtue and excellence. Consequently, virtue must be a mean in the sense that it aims at the mean.
A further proof — it has often been said that in all things there are many ways to do wrong but only one way to do right. The Pythagoreans claim that good is determinate (limited) and evil indeterminate (unlimited), making it easy to do evil and hard to do good. In our view, which is quite similar, there is a single mean surrounded by excessive or deficient alternatives, so that it is easier to find the extremes than the middle. The old saying, "Bad men have many ways, good men but one," is an empirical observation that supports this construct.
It is now possible to offer a more precise definition — Virtue is a disposition of the soul in which, when it has to choose among actions and feelings, it observes the mean relative to itself. this mean is determined by a rational principle of the kind that would be formulated by a man of good sense and practical wisdom.
The mean can always be determined by reference to the two vices of excess and deficiency. All vices exceed or fall short of what is required by virtue in emotion and action, but virtue always finds and chooses the mean. By definition of its nature or essence, virtue is a mean, but in regard to general standards of what is right and good, virtue is an extreme.
We must note that it is impossible to choose a mean in regard to some actions or feelings. Emotions like malice and envy, actions like adultery, theft, and murder are evil in any form or degree. One can never do right by experiencing such feelings or committing such deeds. It is absurd to discuss whether there is a mean, excess, or deficiency in unjust, cowardly, or intemperate actions and emotions, for then one would end up with such ridiculous conceptions as a mean of excess, a mean of deficiency, an excess of excess, or a deficiency of deficiency. In the same way there can be no mean, excess, or deficiency in regard to such things as justice and temperance, for in a sense their mean is an extreme. Stated in general terms, there can be no mean in something which is an excess or deficiency, and there can be no excess or deficiency in a mean. Evil is evil in every form or degree; good is good in every form or degree.