Next, they do produce something.

On the one hand, wisdom produces happiness, not in the sense in which medicine produces health, but in the sense in which health produces health; 164 that is to say, wisdom being a part of complete virtue, its possession and exercise make a man happy.

On the other hand [in the sphere of action], man performs his function perfectly when he acts in accordance with both prudence and moral virtue; for while the latter ensures the rightness of the end aimed at, the former ensures the rightness of the means thereto.

The fourth 165 part of the soul, the vegetative part, or the faculty of nutrition, has no analogous excellence; for it has no power to act or not to act.