"When men have realized that time has upset many fighting faiths," U.S. Supreme Court Justice Oliver Wendell Holmes wrote in a famous dissenting opinion in 1919, "they may come to believe ... 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."
Like any market, however, the marketplace of ideas needs regulation: In particular, its participants should be bound by norms of honesty, humility and civility. Moreover, every idea-trader should adhere to these principles.
Of course, politicians through the ages have polluted the marketplace of ideas with invective. But in American politics, surprisingly, there has been progress.