"Congress shall make no law ... abridging the freedom of speech ...."
— First Amendment

Does freedom of speech protect hate speech? Or fighting words that incite violence? Or even pornography? Even though Americans view freedom of speech as central to the American way -- Supreme Court Justice Benjamin Cardozo wrote that free speech is "the indispensable condition of nearly every other freedom" -- they disagree about how far that freedom goes.

Real debate about freedom of speech began during World War I when Congress passed laws aimed at silencing anti-war protesters. The Supreme Court sided with the government that free speech could be restricted in some circumstances, such as wartime. Justice Oliver Wendell Holmes made an argument in a 1919 case, Schenk v. United States, that was cited in subsequent cases for decades.

"The most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre and causing a panic," Holmes wrote. "The question in every case is whether the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent. It is a question of proximity and degree."

But Holmes almost immediately afterward seemed to sense that his view was wrong. Later the same year, in another case, Abrams v. United States, the Supreme Court majority used Holmes' "clear and present danger" test in favor of restricting free speech. But Holmes wrote a dissent arguing that a democracy depends on the free exchange of ideas, even ideas that are wildly unpopular or even wrong.

"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," Holmes wrote. "That at any rate is the theory of our Constitution. It is an experiment, as all life is an experiment."

Again in 1927 a majority of the court used "clear and present danger" to uphold a California seditious libel law allowing punishment of speech "inimical to the public welfare, tending to incite crime, disturb the public peace, or endanger the foundations of organized government."

Dissenting in that case, Whitney v. California, Justice Louis D. Brandeis wrote that citizens must have freedom to express their views and hear others' views without fear of punishment in order to make democracy work. He said "the greatest menace to freedom is an inert people" and called public discussion a political duty.

"To justify suppression of free speech there must be reasonable ground to fear that serious evil will result if free speech is practiced," Brandeis wrote. "There must be reasonable ground to believe that the danger apprehended is imminent. There must be reasonable ground to believe that the evil to be prevented is a serious one."

In 1969, the court finally ruled that governments cannot prosecute people for advocating ideas the majority condemned as subversive. At the height of raging debate about the Vietnam War, the justices decided that an Iowa school could not punish students for wearing black armbands to class in protest of the war.

Twenty years later the Court ruled in Texas v. Johnson that even the hateful act of burning the American flag was symbolic political speech protected by the First Amendment.

"The flag is constant in expressing beliefs Americans share, beliefs in law and peace and that freedom which sustains the human spirit," Justice Anthony Kennedy wrote. "The case here today forces recognition of the costs to which those beliefs commit us. It is poignant but fundamental that the flag protects those who hold it in contempt."