“What should we think of someone who never admits error, never entertains doubt but adheres unflinchingly to the same ideas all his life, regardless of new evidence? Doubt and skepticism are signs of rationality. When we are too certain of our opinions, we run the risk of ignoring any evidence that conflicts with our views. It is doubt that shows we are still thinking, still willing to reexamine hardened beliefs when confronted with new facts and new evidence.”