The "Idiocy" of an Open Mind

C. S. Lewis:

An open mind, in questions that are not ultimate, is useful. But an open mind about the ultimate foundations either of Theoretical or Practical Reason is idiocy. If a man's mind is open on these things, let his mouth at least be shut. He can say nothing to the purpose. Outside [natural law] there is no ground for criticizing either [natural law] or anything else.

—C. S. Lewis, The Abolition of Man (New York: HarperCollins, 2001), 48.