The Nicomachean Ethics is a philosophical inquiry into the nature of the good life for a human being. Aristotle begins this work by positing that there exists the supreme Good toward which, in the final analysis, all human actions ultimately aim. The characteristics of this Good are that it is complete, final, self-sufficient, and continuous. The Good toward which all human actions implicity or explicitly aim is "eudaimonia", which is often translated as happiness, blessedness or living well, and which is a type of activity. A person who is eudaimon is not simply enjoying life, but is enjoying life by living successfully.
Aristotle treats human happiness as an activity of the soul according to reason. This activity is expressed through ethical virtue. The very highest human life, however, consists in the contemplation of the Greek word arête which is often translated as virtue or excellence. Aristotle divided up virtues into two main kinds: intellectual and moral. A person learns intellectual virtues by instruction, and moral virtues learn by habit and constant practice. There are virtues of character: courage (andreia), moderation (sophrosyne), generosity (eleutheriotes), munificence (megaloprepeia), magnanimity (megalopsuchia), mildness (praotes), and justice (dikaiosyne), and others dealing with sociability. Intellectual virtues are scientific knowledge (episteme), craftsmanship (techne), prudence (phronesis), intelligence (nous), wisdom (sophia), understanding (synesis), and sense (gnome).
Aristotle divides the soul into an irrational and a rational part. The irrational part of the soul has two aspects: the vegetative aspect, which deals with nutrition and growth and has little connection to virtue and the appetitive aspect, which governs our impulses. The rational part of the soul controls these impulses. Because of that, a virtuous person with greater rationality is better able to control his impulses.