Terminiello v. Chicago
334 US 1 (1949)

"The vitality of civil and political institutions in our society depends on free discussion.

As Chief Justice Hughes wrote in DeJonge v Oregon, 299 US 365, it is only through free debate and free exchange of ideas that government remains responsive to the will of the people and peaceful change is effected. The right to speak freely and to promote diversity of ideas and programs is therefore one of the chief distinctions that sets us apart from totalitarian regimes...

Accordingly a function of free speech under our system of government is to invite dispute. It may indeed best serve its high purpose when it induces a condition of unrest, creates dissatisfaction with conditions as they are, or even stirs people to anger. Speech is often provocative and challenging. It may strike at prejudices and preconceptions and have profound unsettling effects as it presses for acceptance of an idea.

That is why freedom of speech, though not absolute, is nevertheless protected against censorship or punishment, unless shown likely to produce a clear and present danger of a serious substantive evil that rises far above public inconvenience, annoyance, or unrest.

There is no room under our Constitution for a more restrictive view. For the alternative would lead to standardization of ideas either by legislatures, courts, or dominant political or community groups." Justice Douglas, U.S. Supreme Court