“When men have realized that time has upset many fighting faiths, they may come to believe... that the ultimate good desired is better reached by free trade in ideas-- that the best test of truth is the power of the thought to get itself accepted in the competition of the market, and that truth is the only ground upon which their wishes safely can be carried out. That at any rate is the theory of our Constitution. It is an experiment. As all life is an experiment. Every year if not every day we wager our salvation upon some prophecy based upon imperfect knowledge. ”