Every virtue is a mean between two extremes, each of which is a vice.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“That monster, custom, who all sense doth eat, Of habits devil, is angel, yet in this, That to the use of actions fair and good He likewise gives a frock or livery That aptly is put on. We now come to the famous doctrine of the golden mean. Every virtue is a mean between two extremes, each of which is a vice. This is proved by an examination of the various virtues. Courage is a mean Between cowardice and rashness; liberality, between prodigality and meanness; proper pride, between vanity and humility; ready wit, between buffoonery and boorishness;” source