structural:summarization
Harm principle

The belief that freedom of speech would advance society presupposed a society sufficiently culturally and institutionally advanced to be capable of progressive improvement. If any argument is really wrong or harmful, the public will judge it as wrong or harmful, and then those arguments cannot be sustained and will be excluded. Mill argued that even any arguments which are used in justifying murder or rebellion against the government should not be politically suppressed or socially persecuted. According to him, if rebellion is really necessary, people should rebel; if murder is truly proper, it should be allowed. However, the way to express those arguments should be a public speech or writing, not in a way that causes actual harm to others. Such is the harm principle: "That the only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others."

At the beginning of the 20th century, Associate justice Oliver Wendell Holmes Jr. made the standard of "clear and present danger" based on Mill's idea. In the majority opinion, Holmes writes:

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.

Holmes suggested that falsely shouting out "Fire!" in a dark theatre, which evokes panic and provokes injury, would be such a case of speech that creates an illegal danger. But if the situation allows people to reason by themselves and decide to accept it or not, any argument or theology should not be blocked.

Here is Mill on the same topic: "No one pretends that actions should be as free as opinions. On the contrary, even opinions lose their immunity, when the circumstances in which they are expressed are such as to constitute their expression a positive instigation to some mischievous act. An opinion that corn-dealers are starvers of the poor, or that private property is robbery, ought to be unmolested when simply circulated through the press, but may justly incur punishment when delivered orally to an excited mob assembled before the house of a corn-dealer, or when handed about among the same mob in the form of a placard" (On LIberty, chapter 3).

Mill's argument is now generally accepted by many democratic countries, and they have laws at least guided by the harm principle. For example, in American law some exceptions limit free speech such as obscenity, defamation, breach of peace, and " fighting words ".