Moderation in all things

From Wikipedia, http://en.wikipedia.org/wiki/Golden_mean_(philosophy)

In philosophy, especially that of Aristotle, the golden mean is the desirable middle between two extremes, one of excess and the other of deficiency.

Nicomachean Ethics has a lot on this subject.

We see the phrase “temperate in all things” more than once scripturally (1 Corinthians 9:25, Alma 7:23, Alma 38:10).

The author of "In Praise of Doubt" describes relativism as too much doubt and fundamentalism as too little doubt.