We tend to blame our failures on gaps in knowledge, when the real culprit is often our certainty about things that are simply wrong—a far more insidious problem because we never think to question them. A confident investor who "knows" a particular stock is undervalued might miss warning signs that contradict his conviction, while someone merely uncertain would stay alert. The trouble isn't ignorance; it's the armor of false confidence that keeps us from learning. Twain's wisdom stings because it suggests that some of our worst mistakes come not from what we're missing, but from what we've already decided we've found.