Source: Urban, Tim. What's Our Problem?: A Self-Help Book for Societies (pp. 519-521).
Popper's Paradox
In his 1945 book The Open Society and its Enemies, philosopher Karl Popper describes a "Paradox of Tolerance" like this:
If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them.
This is only part of Popper's Paradox, but it's the part that's most widely referenced—often quoted more colloquially as, "In order to maintain a tolerant society, the society must be intolerant of intolerance."
The problem here is that tolerance, in itself, is not a principle. "Tolerance" and "intolerance" only take on moral meaning when you add on the "of ____." If the blank is, say, "people who look different than you," then tolerance sounds great. If instead the blank is, "a religious practice that involves sacrificing children," then intolerance suddenly sounds a lot better.
When you leave the "of ___" unspecified, Popper's Paradox is inevitably twisted by political or religious groups into some version of: In order to maintain a tolerant society, the society must be intolerant of [people, ideas, and practices that we don't like]. By labeling their ideological opponents as "intolerant," whoever has the most cultural power in any environment can use Popper's reasoning to justify authoritarianism.
The Economist points out that every authoritarian who suppresses free speech justifies it as a means of protecting people:
Nearly all countries have laws that protect freedom of speech. So authoritarians are always looking out for respectable-sounding excuses to trample on it. National security is one. Russia recently sentenced Vadim Tyumentsev, a blogger, to five years in prison for promoting "extremism", after he criticised Russian policy in Ukraine. "Hate speech" is another. China locks up campaigners for Tibetan independence for "inciting ethnic hatred"; Saudi Arabia flogs blasphemers; Indians can be jailed for up to three years for promoting disharmony "on grounds of religion, race...caste...or any other ground whatsoever."
Each of these cases are essentially framed as "being intolerant of intolerance."
But if we read Popper's Paradox in its fuller context, we see that he's talking about something very specific:
I do not imply, for instance, that we should always suppress the utterance of intolerant philosophies; as long as we can counter them by rational argument and keep them in check by public opinion, suppression would certainly be most unwise. But we should claim the right to suppress them if necessary even by force; for it may easily turn out that they are not prepared to meet us on the level of rational argument, but begin by denouncing all argument; they may forbid their followers to listen to rational argument, because it is deceptive, and teach them to answer arguments by the use of their fists or pistols. We should therefore claim, in the name of tolerance, the right not to tolerate the intolerant.
Popper was specifically concerned by those who refuse to engage in rational argument, instead using intimidation to respond to criticism of their ideas. When he talked about intolerance, he was referring to one kind in particular: idea supremacy. Popper believed that liberal societies have to be intolerant when people impede the workings of the marketplace of ideas. Not only is SJF idea supremacy not justified by Popper's Paradox, it is exactly what Popper was warning about.