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).
