First of all, it must be observed that the nature of moral qualities is such that they are destroyed by defect and by excess. We see the same thing happen in the case of strength and of health, to illustrate, as we must, the invisible by means of visible examples: excess as well as deficiency of physical exercise destroys our strength, and similarly, too much and too little food and drink destroys our health; the proportionate amount, however, produces, increases, and preserves it. The same applies to self-control, courage, and the other virtues: the man who shuns and fears everything and never stands his ground becomes a coward, whereas a man who knows no fear at all and goes to meet every danger becomes reckless. Similarly, a man who revels in every pleasure and abstains from none becomes self-indulgent, while he who avoids every pleasure like a boor becomes what might be called insensitive. Thus we see that self-control and courage are destroyed by excess and by deficiency and are preserved by the mean. (II,1104a)
Aristotle. Nicomachean Ethics. Trans. Martin Ostwald. Prentice Hall, 1999.