Aristotle proposes and defends a definition of virtue in Book II of the Nicomachean Ethics. He concludes his inquiry by saying, "Virtue, then, is a state involving rational choice, consisting in a mean relative to us and determined by reason - the reason, that is, by reference to which the practically wise person would determine it" (Crisp, 35). Each part of his definition is fundamental to his conception of virtue.
First, virtue is not a capacity or a feeling, but rather a state. Merely being able to be virtuous is not sufficient for one to be virtuous. While a capacity for virtue may exist, this is not virtue itself. Nor is virtue a feeling. It is not excellent to desire a particular thing. One is only virtuous if one is the sort of person who acts virtuously. Hence virtue must be a state, according to Aristotle. This state comes into being through the practice of virtue. It is by doing virtuous things that one becomes virtuous.
Next, virtue involves rational choice. It is not enough to accidentally do something virtuous. For an action to be virtuous, it must be knowledgeably chosen for its own sake. The choice is oriented to action, so it involves practical reasoning. This is why virtue involves reason of the sort by which a the practically wise person would choose an action.
Finally, virtue is a mean relative to us. For each virtue, there are two vices. On the one hand, there are deficiencies such as cowardice, niggardliness, stinginess, or self-deprecation. On the other hand, there are excesses such as rashness, vulgarity, wastefulness, or boastfulness. Between excess and deficiency stands the virtue, the mean. These include courage, magnificence, generosity, and truthfulness, among other virtues. There are actions for which there is no mean, such as murder and adultery, but virtue is always a mean.