Moral virtue, or excellence of character, is the disposition (Grk hexis) to act excellently, which a person develops partly as a result of his upbringing, and partly as a result of his habit of action. Aristotle develops his analysis of character in Book II of the Nicomachean Ethics, where he makes this argument that character arises from habit—likening ethical character to a skill that is acquired through practice, such as learning a musical instrument. In Book III of the Nicomachean Ethics, Aristotle argues that a person's character is voluntary, since it results from many individual actions which are under his voluntary control.
Aristotle distinguishes the disposition to feel emotions of a certain kind from virtue and vice. But such emotional dispositions may also lie at a mean between two extremes, and these are also to some extent a result of up-bringing and habituation. Two examples of such dispositions would be modesty, or a tendency to feel shame, which Aristotle discusses in NE IV.9; and righteous indignation (nemesis), which is a balanced feeling of sympathetic pain concerning the undeserved pleasures and pains of others. Exactly which habitual dispositions are virtues or vices and which only concern emotions, differs between the different works which have survived, but the basic examples are consistent, as is the basis for distinguishing them in principle.
Some people, despite intending to do the right thing, cannot act according to their own choice. For example, someone may choose to refrain from eating chocolate cake, but finds himself eating the cake contrary to his own choice. Such a failure to act in a way that is consistent with one's own decision is called "akrasia", and may be translated as weakness of will, incontinence, or lack of self-mastery.