Nicomachean Ethics 1.7 (1097b20-1098a26) (The Human Function)(Fred Castiglia, Zachary Cohen)

The end of human life is happiness (well-being), the best good for humans. Aristotle tries to specify what that best good is by examining the question of whether humans qua human have a function.

From Aristotle's examples, we can generalize: if F qua F has a function φ, then well-being for an F is optimal φ-ing.

For Aristotle, actuality is superior to potentiality. Hence the human function is "the soul's activity that expresses reason" (both in the sense of thinking and reasoning, but also in the sense of obeying reason: they are first and second actualities).

Aristotle thinks that the function of anything is not just to do that thing's characteristic activity, but to do it well. Take a knife: it is for cutting, but not just any cutting. It is for cutting well.

You might say that a dull knife is for dull cutting, or that a tone-deaf,  tempo-less, twitchy piano player is for tone-deaf, tempo-less, and jerky piano-playing. Aristotle simply disagrees: things have essences and those essences mean that things have characteristic activities, and doing them well is the real goal of things. So a thing's nature gives it a target, which it often misses, but that is still its target.

Aristotle argues as follows:

1. The human function is the activity of the human soul qua human soul.
2. The differentia of the human soul (that which makes it a human rather than any other soul) is reason.
   
1. What would you substitute for that if you disagree? Would that involve major changes in other things about Aristotelianism?
   
3. Thus by 1 and 2, the activity of the human soul qua human soul is reasoning.
4. There are two senses of reason:
1. Reason qua thinking / reasoning. This is a first actuality.
   
2. Reason qua ordering one's life and acting in accordance with reason (i.e. obeying reason is an expression of reason). This is a second actuality.
   
5. The function of any F is the same in kind as that of an excellent F.
1. By this Aristotle means that the function of a dull knife, for instance, is the same as that of a sharp knife: cutting. The sharp knife does that function excellently, and so we call it an excellent knife.
2. This principle serves to show that good and bad performers of a function are nonetheless performers of the same function.
6. In general the function of an excellent F is to excellently perform the function that is characteristic of F.
   
7. The excellent human's function is to reason excellently (by 3 and 6).
8. The good for F qua F is to be an excellent F.
9. Being an excellent F is performing the characteristic function of F excellently.
   
10. Thus, by 7, 8, and 9, the human good is to reason excellently.