Nicomachean Ethics

And that in medicine is health, in war is victory, in building is a house⁠—a different thing in each different case, but always, in whatever we do and in whatever we choose, the end. For it is always for the sake of the end that all else is done.
If then there be one end of all that man does, this end will be the realizable good⁠—or these ends, if there be more than one.
By this generalization our argument is brought to the same point as before. 10 This point we must try to explain more clearly.
We see that there are many ends. But some of these are chosen only as means, as wealth, flutes, and the whole class of instruments. And so it is plain that not all ends are final.
But the best of all things must, we conceive, be something final.
If then there be only one final end, this will be what we are seeking⁠—or if there be more than one, then the most final of them.
Now that which is pursued as an end in itself is more final than that which is pursued as means to something else, and that which is never chosen as means than that which is chosen both as an end in itself and as means, and that is strictly final which is always chosen as an end in itself and never as means.
Happiness seems more than anything else to answer to this description: for we always choose it for itself, and never for the sake of something else; while honour and pleasure and reason, and all virtue or excellence, we choose partly indeed for themselves (for, apart from any result, we should choose each of them), but partly also for the sake of happiness, supposing that they will help to make us happy. But no one chooses happiness for the sake of these things, or as a means to anything else at all.
We seem to be led to the same conclusion when we start from the notion of self-sufficiency.
The final good is thought to be self-sufficing [or all-sufficing]. In applying this term we do not regard a man as an individual leading a solitary life, but we also take account of parents, children, wife, and, in short, friends and fellow-citizens generally, since man is naturally a social being. Some limit must indeed be set to this; for if you go on to parents and descendants and friends of friends, you will never come to a stop. But this we will consider further on: for the present we will take self-sufficing to mean what by itself makes life desirable and in want of nothing. And happiness is believed to answer to this description.
And further, happiness is believed to be the most desirable thing in the world, and that not merely as one among other good things: if it were merely one among other good things [so that other things could be added to it], it is plain that the addition of the least of other goods must make it more desirable; for the addition becomes a surplus of good, and of two goods the greater is always more desirable.
Thus it seems that happiness is something final and self-sufficing, and is the end of all that man does.
But perhaps the reader thinks that though no one will dispute the statement that happiness is the best thing in the world, yet a still more precise definition of it is needed.
This will best be gained, I think, by asking, What is the function of man? For as the goodness and the excellence of a piper or a sculptor, or the practiser of any art, and generally of those who have any function or business to do, lies in that function, so man’s good would seem to lie in his function, if he has one.
But can we suppose that, while a carpenter and a cobbler has a function and a business of his own, man has no business and no function assigned him by nature? Nay, surely as his several members, eye and hand and foot, plainly have each his own function, so we must suppose that man also has some function over and above all these.
What then is it?
Life evidently he has in common even with the plants, but we want that which is peculiar to him. We must exclude, therefore, the life of mere nutrition and growth.
Next to this comes the life of sense; but this too he plainly shares with horses and cattle and all kinds of animals.
There remains then the life whereby he acts⁠—the life of his rational nature, 11 with its two sides or divisions, one rational as obeying reason, the other rational as having and exercising reason.
But as this expression is ambiguous, 12 we must be understood to mean thereby the life that consists in the exercise of the faculties; for this seems to be more properly entitled to the name.
The function of man, then, is exercise of his vital faculties [or soul] on one side in obedience to reason, and on the other side with reason.
But what is called the function of a man of any profession and the function of a man who is good in that profession are generically the same,  e.g. of a harper and of a good harper; and this holds in all cases without exception, only that in the case of the latter his superior excellence at his work is added; for we say a harper’s function is to harp, and a good harper’s to harp well.
(Man’s function then being, as we say, a kind of life⁠—that is to say, exercise of his faculties and action of various kinds with reason⁠—the good man’s function is to do this well and beautifully [or nobly]. But the function of anything is done well when it is done in accordance with the proper excellence of that thing.) 13
If this be so the result is that the good of