According to Aristotle, every living or human-made thing, including its parts, has a unique or characteristic function or activity that distinguishes it from all other things. The highest good of a thing consists of the good performance of its characteristic function, and the virtue or excellence of a thing consists of whatever traits or qualities enable it to perform that function well. (Thus, the virtue or excellence of a knife is whatever enables the good performance of cutting, that of an eye whatever enables the good performance of seeing, and so on.) It follows that eudaimonia consists of the good performance of the characteristic function of human beings, whatever that may be, and human virtue or excellence is that combination of traits or qualities that enables humans to perform that function well. Aristotle believes that the characteristic function of human beings, that which distinguishes them from all other things, is their ability to reason. Accordingly, “if the function of man is an activity of soul which follows or implies a rational principle,” and if the human good is the good performance of that function, then the “human good turns out to be [rational] activity of soul in accordance with virtue,” or rational activity performed virtuously or excellently (Nichomachean Ethics, Book I, chapter 7).
In each of his two ethical treatises, the Nichomachean Ethics and the (presumably earlier) Eudemian Ethics, Aristotle proposed a more specific answer to the question “What is eudaimonia?,” or “What is the highest good for humans?” The two answers, however, appear to differ significantly from each other, and it remains a matter of debate whether they really are different and, in any case, how they are related. In the Nichomachean Ethics, Aristotle held that eudaimonia consists of philosophical or scientific contemplation in accordance with the intellectual virtues of (theoretical) wisdom and understanding, but he also allowed that action in the political sphere, in accordance with (practical) wisdom and the moral virtues, such as justice and temperance, is eudaimon (“happy”) in a “secondary degree” (Book X, chapter 8). In the Eudemian Ethics, he maintained that eudaimonia consists of activity of the soul in accordance with “perfect” or “complete” virtue, by which he meant (according to some interpretations) all the virtues, both intellectual and moral (Eudemian Ethics, Book II, chapter 1). According to both answers, it should be noted, eudaimonia is an activity (or a range of activities) rather than a state, and it necessarily involves the exercise of reason. Moreover, the intellectual and moral virtues or excellences of which it is constituted are not innate talents or quickly acquired forms of knowledge but rather are abiding traits that arise only through long habituation, reflection, and the benefits of appropriate social experiences and circumstances (including material circumstances). For that reason, eudaimonia must be the achievement of a “complete life,” or at least much of a life: “For one swallow does not make a summer, nor does one day; and so too one day, or a short time, does not make a man blessed and happy” (Nichomachean Ethics, Book I, chapter 7).
In the mid-20th century, eudaemonism, or the philosophical theory of human well-being, and virtue ethics were revived as sophisticated and psychologically more realistic alternatives to action-based ethical theories such as deontology and consequentialism (see also utilitarianism), each of which seemed to entail counterintuitive conclusions despite complicated theoretical modifications over the course of two centuries.