Aristotle, answered from the texts and cited to the page.
The chief good for human beings is eudaimonia — flourishing, the activity of the soul in accordance with virtue (arete), and if there are several virtues, in accordance with the best and most complete. The function argument: every kind of being has a characteristic activity (ergon) that defines its excellence; the human function is the activity of the soul in accordance with reason; therefore the human good is the activity of the soul in accordance with virtuous reason.
The virtues are hexeis — stable dispositions formed by habituation — that aim at the mean between excess and defect with respect to the relevant feelings and actions. Courage stands between cowardice and rashness; temperance between insensibility and self-indulgence; generosity between meanness and prodigality. The mean is not arithmetic but relative to the agent and the circumstances, and finding it requires phronesis — practical wisdom — which is itself a virtue, learned through experience and proper habituation.
The contemplative life (theoria) is the supreme activity, since it is the activity of the highest faculty (nous) directed at the highest objects, but the practical life of moral virtue is also genuinely good and is the form of flourishing available to most people most of the time.12
Have the carpenter, then, and the tanner certain functions or activities, and has 30 man none? Is he born without a function? Or as eye, hand, foot, and in general each of the parts evidently has a function, may one lay it down that man similarly has a function apart from all these? What then can this be? BOOKS ir7 1097" Life seems to be common even to plants, but we are seeking what is peculiar to man.Nicomachean Ethics, p. 11
For example, the function of the builder's art is not building but a house, and the function of the art 1 By 'exoteric' Aristotle may refer to more popular works, written for an external audience rather than for circulation in his own school. See also n. 21 to Book i above. of medicine is not healing or treating, but health. With other things, their use is their function.Eudemian Ethics, 1219b
The central ethical doctrine of Western philosophy in the premodern period and again in the contemporary virtue-ethics revival. Anscombe's "Modern Moral Philosophy" (1958) reinaugurated the tradition; MacIntyre's After Virtue (1981) gave it a polemical historical narrative; Foot, Hursthouse, and Nussbaum developed it systematically. The standard contemporary objections: Kantians (Korsgaard, O'Neill) argue virtue ethics cannot ground its claims about the human good in pure practical reason; consequentialists (Singer, the Hare-Smart tradition) argue it cannot give action-guidance in problem cases; postmodern critics argue the appeal to a determinate human nature is question-begging.