"When I was fourteen, my father was so ignorant I could hardly stand to have him around. When I got to be twenty-one, I was astonished at how much he had learned in seven years. See what happens when you "know it all", at any stage of life? Farther down the track you may see clearly how certain personal opinions, held onto too tightly, could be fogging up the view, and providing incorrect insight. Prosperity is the best protector of principle."

Source: Attributed in Reader's Digest, Sept. 1937. The attribution to Twain is obviously spurious because Twain's father died when the future writer was eleven years old.