Please excuse the dated gender references. Feel free to add more appropriate quotes in the comments….

“It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.” — Mark Twain

“…the mark of a free man is that ever-gnawing inner uncertainty as to whether or not he is right.” — Judge Learned Hand

“[A community organizer] must constantly examine life, including his own, to get some idea of what it is all about, and he must challenge and test his own findings…To the extent that he is free from the shackles of dogma, he can respond to the realities of the widely different situations our society presents.” — Saul Alinsky

“The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts.” — Bertrand Russell

“Doubt is not a pleasant condition, but certainty is absurd.” — Voltaire