Here are some notes on the reasons by which, in my philosophical reading so far this summer, free speech is justified. This is very vague and is in no way intended to be comprehensive,  coherent, or particularly clear:

  • Human progress in general

    • Questioning ideas is necessary in order to improve and refine them. If we foreclose any discussions, we will necessarily foreclose some possibilities of progress toward the truth. More generally, we will also produce an intellectually stifling atmosphere.

    • It’s important to be able to discuss even bad and wrong ideas, since this improves our own understanding of, and ability to defend, the good. (For cases in which censors are concerned with promoting virtuous behavior, it can be argued that is necessary that the good be chosen over an alternative in order for the behavior to be good.)

    • Censorship in general, in addition to creating an intellectually stifling atmosphere, creates contempt for the laws as censorship must necessarily be somewhat arbitrary.

  • Practicality (cf. the highly impractical plan set forth in The Republic).

    • Censoring bad speech does nothing to defeat it. Though human beings can to a limited extent be forced to behave certain ways, by their very nature, human beings can’t be forced to think certain things. Even if they could, we don’t have the technology to verify this.

    • While it could be argued that censorship can prevent bad ideas or bad speech from spreading or, say, “corrupting the youth”, in practice censoring ideas requires calling attention to them (Socrates, Jesus, many others), and therefore exacerbating the problem.

    • Once a thought has been formulated and expressed, it is impossible to actively expurgate it - a single copy of a book, for example, a remembered slogan, or something even more ephemeral will be enough to revive it. Or, the thought or idea can spontaneously reemerge somewhere else. In general, effective censorship would require a conspiracy vast, eternal and unerring.

    • In order to censor speech, it is necessary to expose the censors themselves to the ideas. It is impossible to ensure that the censors themselves will not be influenced.

    • Any standards of censorship must necessarily be arbitrary. This makes it impossible to enforce a standard of censorship consistently, resulting in (a) the occasional frustration of good speech, and (b) the occasional failure to frustrate bad speech. While it is true that there is some arbitrariness in any human system, censorship does not rely on establishing facts (as might happen in a prosecution for theft - did the theft occur or not?), but instead on establishing a particular interpretation of an intellectual work.

  • Justice

    • People should be able to do whatever they want, provided it does not harm others. This is the embodiment of the Golden Rule, and the only possibly consistent standard to govern human behavior. Offense, which is the only harm that can result from most types of speech, is completely under the control of the hearer and therefore is no harm at all.

    • Seeking and imparting information and ideas is vital for pursuing happiness, and for creating meaning in our lives. Humans are social animals who depend for their happiness on unfettered association and discussion with others.

    • Because censorship must favor certain speech over other speech, its prohibitions will necessarily fall unequally on people with different ideas, beliefs, or value systems. This violates the idea that all people should be equal before the law.

    • The ability to speak freely is necessary for all other political rights, and compatible with all activities necessary to ensure justice.

These are just what I’ve been able to remember from my reading, though I think I’ve covered a lot of the given reasons. There would be lots to discuss here - certainly, speech examples could be imagined to which many, or perhaps all, of the above reasons wouldn’t apply.

Also, as far as concerns practicality, it could be argued that many censorship regimes have been maintained over long periods. However, I would note that this tends to require increasing repression over time, which creates problems under the Justice and Human Progress headings.

Finally, it seems to me that many of these arguments could also apply to other types of freedom, such as economic freedom. Could you substitute “preventing transactions” or “preventing agreements” for “censoring ideas”?