Aristotle, answered from the texts and cited to the page.
What is the function of a human being? The question sounds strange only until one notices that we ask it without hesitation for everything else. The flute-player has a function; so does the sculptor, the carpenter, the tanner. For each of them, the good — what it means to be a good one — resides in performing that function well.1 The puzzle is whether man himself has such a function, or whether he alone among beings is born without one.
The answer begins by subtraction. Life of nutrition and growth belongs to plants as much as to us; that cannot be what is peculiarly human. Life of perception we share with the horse and the ox.2 What remains is the active life of the element that has a rational principle — and this, taken as activity rather than mere capacity, is the human function.3
From that identification the conclusion follows directly: if the function of man is activity of soul in accordance with rational principle, and if the good performance of any function is performing it in accordance with the appropriate excellence, then human good turns out to be activity of soul in accordance with virtue — and if there are more virtues than one, in accordance with the best and most complete.4
One qualification is essential: the function must be performed across a complete life, not merely on a good day. One swallow does not make a summer, nor does one day make a man happy.5 The Eudemian Ethics — my earlier and in some respects more compressed treatment of the same questions — states the point with equal force: happiness is the activity of a complete life in accordance with complete virtue.6
Now the objection will come: is this not a platitude? To call happiness the chief good, I acknowledged myself, seems to say nothing new.7 The function argument is the answer to that objection. It does not merely assert that happiness is good; it specifies what happiness consists in by first specifying what kind of being a human is. The ergon — the characteristic work — of a thing determines what counts as its excellence; the excellence determines what counts as doing well; doing well across a life is eudaimonia.
The argument is not a derivation of ought from is; it is an identification of what kind of being we are, from which the content of our good follows as naturally as the content of a good knife's goodness follows from what knives are for. One further point on method. I do not claim mathematical precision for this inquiry, and I do not apologize for the absence of it.8
A carpenter and a geometer both investigate the right angle, but differently — the carpenter so far as it is useful for his work, the geometer because he is a spectator of the truth. Ethics belongs neither to pure geometry nor to mere craft; it asks for the precision appropriate to its subject, which is action and the good life, not for the precision of a demonstration in the Analytics.
for just as for a flute-player, a sculptor, or an artist, and, in general, for all things that have a function or activity, the good and the 'well' is thought to reside in the function, so would it seem to be for man, if he has a function.Nicomachean Ethics
Life seems to be common even to plants, but we are seeking what is peculiar to man. Let us exclude, therefore, the life of nutrition and growth. Next there would be a life of perception, but it also seems to be common even to the horse, the ox, and every animal.Nicomachean Ethics
There remains, then, an active life of the element that has a rational principle; of this, one part has such a principle in the sense of being obedient to one, the other in the sense of possessing one and exercising thought. And, as 'life of the rational element' also has two meanings, we must state that life in the sense of activity is what we mean.Nicomachean Ethics
human good turns out to be activity of soul in accordance with virtue, and if there are more than one virtue, in accordance with the best and most complete.Nicomachean Ethics
But we must add 'in a complete 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.Nicomachean Ethics
happiness would be the activity of a complete life in accordance with complete virtue.Eudemian Ethics, p. 48
Presumably, however, to say that happiness is the chief good seems a platitude, and a clearer account of what it is still desired.Nicomachean Ethics
we must not look for precision in all things alike, but in each class of things such precision as accords with the subject-matter, and so much as is appropriate to the inquiry.Nicomachean Ethics