Paradox of tolerance

When I am weaker than you, I ask you for freedom because that is according to your principles; when I am stronger than you, I take away your freedom because that is according to my principles. The Paradox of Tolerance is a concept advanced by the philosopher Karl Popper which claims that unlimited tolerance necessarily results in the destruction of the tolerant by the intolerant, resulting in a society in which tolerance is no longer possible. Therefore, while paradoxical to the concept of free speech, it is necessary to be intolerant of intolerance. The concept is important in discussions on free speech, its limits (if they exist), and to whom the right to speak must be afforded — generating endless controversy and bad arguments from people of all colours of the political spectrum.

What Popper actually said
There is a degree of misunderstanding regarding the tolerance paradox, since Popper is not always quoted in full. This is his 1945 statement:

Effectively, some people are prepared to abandon the realm of logic and reason, instead turning to violence. If society tolerates violence for tolerance's sake, the result is that this society engenders its own extinction. With the demise of the tolerant, the bigots and hate preachers of society will prevail. Therefore, in his opinion, it is valid to suppress such agitators before they take advantage of and destroy the society that extended them the benefit of the doubt (effectively stopping them from biting the hand that fed them, by stopping them feeding at all). This is why even in countries that allow freedom of expression to a liberal degree, there are some restrictions, such as the incitement of violence. A good example would be the radical Islamic cleric Anjem Choudary, who was jailed in the UK for violent speech.

It makes sense, doesn't it? Free speech is all fine and dandy, but let's stretch that to the limit. A and B are promoting their ideologies. A-ism is based on reasoned arguments — they may not yield correct conclusions, or they may, but A is speaking in good faith. B-ism is based on calls to violence and insurrection. If both are afforded the right to speak freely, modelling things out, B will necessarily inflict violence, or threats of such, on A — but violence and violent threats have the effect of silencing others, which indirectly impedes their right to speak freely — you are not 'free' to speak if someone will hurt you for doing so! Thus, free will is replaced with coercion, and society suffers as a result.

Misuse
Unfortunately, the name of the concept has made it ripe for abuse and misuse by moonbats and wingnuts alike. Some on the right use similar logic to the "everyone is racist" argument, stating that because no one can be perfectly tolerant, the concept of tolerance is tenuous to begin with, and this gives them free reign to oppress groups that don't align with their ideal society — namely women and ethnic minorities (this becomes especially true in the case of white nationalists). "Everyone is intolerant, at least we admit it," they might claim, which confuses internal consistency with rationality — simply having a consistent moral framework doesn't mean that those morals are good. In addition, there is an argument for pre-emptive suppression of groups that are likely to turn violent — the alt-right, for example, may not be consistently violent, but there has been an uptick in attention paid to right-wing terrorism recently. Should we tolerate at the cost of lives? (Or is that a false dilemma?)

Likewise, many liberals and others on the left make the argument that because of the paradox of tolerance, intolerant views cannot be tolerated, and this is thus to be used as a defence against intolerant views. The keyword here — intolerant — being however they choose to define it, making for some interesting takes, to say the least. However, it too is based on a fallacy if used as an argument for censorship, since Popper explicitly states that he considered such laws to be unwise. In defence of deplatforming, Popper is often quote-mined to suggest that the default position on intolerance is suppression, when this really only applies to violence (for which the definition and extent are up for debate). And ironically enough, given that some communists argue for 'violent revolution' and joke about 'killing/eating' the rich, this actually hurts them as well as the far-right.

Joseph Goebbels
Nazi Minister of Propaganda Goebbels was well aware of this paradox as early as 1935 and exploited it to the Nazi Party's advantage: This will always remain one of the best jokes of democracy, that it gave its deadly enemies the means by which it was destroyed. (Das wird immer einer der besten Witze der Demokratie bleiben, daß sie ihren Todfeinden die Mittel selber stellte, durch die sie vernichtet wurde.)

The point?
Free speech, like any other right, ends where other rights begin.