Earlier quoted context omitted.
The fact that a reason is used (and has been used) to justify censorship unjustly does not mean that the reason itself is invalid, or that there is no such thing as good censorship; most people agree that some censorship (of threats or child pornography etc.) can be a great positive force. There is no reason behind the idea that because we can't totally differentiate between good and bad where the line is blurry then…
The debate isn't whether we can do any sort of "good" censoring, but whether we should do any censoring at all. (Just to be clear, I'm narrowing the scope under discussion to ideas. Of course things like child pornography should be censored due to the direct harm.) I reject the idea that society should welcome some authority having control over ideas such that ones deemed "bad" enough by a large enough majority shoul…
Here we see the vacuity of the harm principle: one can claim anything is (or isn't) harmful in order to attach their favorite idea to it. As an example, some people may be said to be harmed simply by the knowledge that someone is watching pornography in their house. You'd likely say that doesn't "count" as harm - well then what does? As it turns out, controlling speech under your schema is simply a matter of defining what counts as harm and what doesn't. Yet philosophers such as Joel Feinberg and Catherine MacKinnon have pointed out, very few people (if any) would like to live in a society in which only harmful speech (or acts, since there is no meaningful distinction between speech and acts other than invoking body-mind dualism) is not permitted.
Then we get to ideas: who's to say that threats or child porn can't carry ideas in them? In censoring them, aren't we censoring ideas too? Some would say the idea that "it's not so bad to have sex with children" is encoded in every instance of child pornography. What if I made my threat into an art piece?