Why should the value we place on freedom of speech extend to cover insult and ridicule, given that these sorts of speech aren't obviously constructive?

I don't know where you're writing from, but in almost every part of the world, the law does notprotect speech that is insulting or expresses ridicule. This kind of speech is typically classified as defamationhttps://en.wikipedia.org/wiki/Defamation: When written, it's known as libel, and when spoken, slander.

There are some differences in how different nations understand defamation, but in general terms, it's understood as the expression of a false statement, known or believed by the speaker to be false, aimed at harming the reputation of a person or group. This definition suggests that the philosophical rationale for defamation not being legally protected is along the lines you suggest, namely, that it's not "constructive." Generally speaking, we do not much benefit by believing what is false, so the audience for defamatory speech does not benefit from it. Moreover, defamatory speech does not contribute to public discourse and is not even intended to advance our knowledge of the truth. Hence, defamation doesn't seem to have any of the properties that make free speech valuable, and given that it can cause harm, it is difficult to see why the law should protect it.

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