Every person has a function or activity. Aristotle raises this first with reference to a professional such as a flautist, sculptor, or artist. We can see that performing the task that identifies them as a certain type of performer or artist is their function, and individuals in the same grouping would be ranked according to how well they perform this function. This is easy to see with reference to a profession, for it is easy to identify what the person does as an activity and how to judge how well he or she does it. .
The issue is different when considering simply man as an entity, for while these professionals "have certain functions or activities, man a such has none, but has been left by nature a functionless being" (75), or so it may seem. Aristotle is looking for what he calls man's "proper function" (75, so it is not enough to say that man's function is to live, given that this activity is shared by plants. He next considers the idea of sentient life, but man shares that with animals. This leads him to consider "a practical life of the rational part" (75). This leads to the view that "the function of man is an activity of the soul" (75) which accords with or implies "a rational principle" (76).
Continuing his analysis, Aristotle makes an analogy to the professional perfecting the action for which he is a professional, so the human being must seek the perfection of his activity, which is the activity of the soul. Aristotle assumes that the function of man is a certain kind of life, meaning "an activity or series of actions of the soul, implying a rational principle" (76). More specifically, the function of a man is to perform this activity "well and rightly" (76), and to be performed well means "performed in accordance with its proper excellence" (76). .
Aristotle has here developed a series of connected ideas leading from the idea that man has a proper function to a statement of what that proper function would be and how to judge whether it is being performed successfully.
