Yelling fire in a theater expresses the idea that there is a fire in the theater, whether it's true or not. It's a good example because it's a pretty bad and at

Yelling fire in a theater expresses the idea that there is a fire in the theater, whether it's true or not. It's a good example because it's a pretty bad and at best worthless thing to express, and because it has been used before (Schenck v. United States) when free speech has been abridged where the speech in question wasn't categorically not covered, just dangerous and unmerited enough to be considered illegal anyway. It is considered a well motivated abridgment. There's nothing about the constitutional idea of free speech that says it has to be of philosophical value or true.