Holmes: Free Trade In Ideas Is The American Constitutional Theory

Persecution for the expression of opinions seems to me perfectly logical. If you have no doubt of your premises or your power, and want a certain result with all your heart, you naturally express your wishes in law, and sweep away all opposition. To allow opposition by speech seems to indicate that you think the speech impotent, as when a man says that he has squared the circle, or that you do not care wholeheartedly for the result, or that you doubt either your power or your premises. But when men have realized that time has upset many fighting faiths, they may come to believe even more than they believe the very foundations of their own conduct 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.

—Justice Oliver Wendell Holmes dissenting opinion in Abrams v United States (1919)