For Aristotle, an understanding of how to live the good life did not rely on an inductive contemplation of natural laws—as was the case in science—but rather the application of practical reason to action. Unlike scientific principles, whose reality have basis in inductive truth, Aristotle argues that there is no such thing as a single rule of action that determines the good life. As he states in Book II, Chapter 2 of the Nicomachean Ethics, “matters concerned with conduct and questions of what is good for us have no fixity, any more than matters of health.” Rather, for Aristotle, deficiency and excess destroys the nature of the good life and of individual health, and practical action is therefore the maintenance of a balance between these two extremes. This is the foundation of the “golden mean” of Aristotelean ethics.
Aristotle illustrated the idea of the golden mean in his description of a courageous life. An individual, be he a soldier, politician, or private citizen, must exercise a certain degree of martial spirit in his everyday life. However, Aristotle was careful to admonish his readers that the excessive or insufficient exercise of this militant character would be to approach the extremes and thus to sacrifice the good life. An individual who is overly pugilistic, ready to fight at every encounter, is an unbalanced brute. On the other hand, one who is too weak-willed to defend himself in any situation is a coward—and thus similarly unbalanced. According to Aristotle, the courageous man is one capable of fighting battles when necessary but self-controlled enough to engage in other kinds of relationships with his fellow citizens. The courageous man, in other words, embodied the good life, because he achieves a perfect mean between two extremes of personal conduct.