It is the highest part because it is the use of reason that makes us different from the "horse, ox, and every animal." "The rational element has two parts: one is rational in that is obeys the rule of reason, the other in that it possesses and conceives rational rules." What he means is that the use of reason is understood as an action, but acting in a rational way. "The proper function of man, then, consists in an activity of the soul in conformity with a rational principle." .
There are many particulars in gaining this happiness Aristotle speaks of. The human good is the activity of the soul exhibiting excellence for ones entire life. Human excellence is the activity of the soul according to reason. We know this because of the human function. It is not just having the ability to be virtuous. It is the activity, not just possessing it. "The function of the harpist is to play the harp; the function of the harpist who has high standards is to play it well. It is excellence that makes men capable of performing noble deeds." Arete is that quality which enables its possessor to perform his own function well. Virtue is not a part of the soul. "Virtue or excellence renders good the thing itself which it is the excellence, and causes it to perform its function well. "The higher the virtuous activities, the more durable they are, because men who are supremely happy spend their lives in these activities most intensely and most continuously, and this seems to be the reason why such activities cannot be forgotten." There are two types of virtues: 1) intellectual, and 2) moral. Intellectual virtue owes its origin and development chiefly by teaching. Intellectual activity may be gained by work, experience, and learning. Moral is formed by habit, but Aristotle points out that nothing that exists by nature can be formed by habit. Aristotle says good habits should be formed early, because it is extremely difficult to change one's habits after they are formed.