A fad or heresy is the exaltation of something which even if true, is secondary or temporary in its nature against those things which are essential and eternal, those things which always prove themselves true in the long run. In short, it is the setting up of the mood against the mind.

The Quotable Chesterton: A Topical Compilation of the Wit, Wisdom and Satire of G.K. Chesterton (ed. 1986)