Eudaimonia is not a subjective state — a pleasant feeling or a sense of contentment. It is a mode of activity: the active exercise of one's capacities in accordance with virtue. This is why Aristotle insists it requires a complete lifetime. A single day of excellent action does not make a person happy in his sense, any more than one swallow makes a spring.
Aristotle surveys the popular candidates for the good life and finds each wanting. The life of pleasure makes us no different from cattle — it mistakes a means of living for the point of living. The life of honour depends too much on those who bestow it and cannot therefore be truly one's own. The life of wealth is plainly a means to other things, not an end in itself.
The serious candidate is virtue — and Aristotle agrees that virtue is central. But he argues that virtue alone, without its exercise, is insufficient. A person of perfect character who is asleep, or prevented by misfortune from acting, is not yet happy in the full sense. Happiness requires that virtue be lived, not merely possessed.
Aristotle is honest about happiness's vulnerability. External goods — health, friendship, reasonable prosperity, even good looks — are genuinely necessary conditions for the full good life. Someone who loses all their children, or lives in extreme poverty, or suffers severe disfigurement, cannot be called fully happy however virtuous they are. The good life is not invulnerable to fortune.
What makes the concept of eudaimonia enduringly important is its insistence that happiness is an achievement rather than a gift. It is not something that happens to us but something we do — something we can do better or worse, something that requires cultivation, practice, and the right kind of community. This makes ethics a practical discipline, not merely a theoretical one.
Eudaimonia is introduced and analysed across Books I and X of the Nicomachean Ethics, with Book I establishing the concept and Book X returning to it in the context of the contemplative life.