The paradox of tolerance arises when a tolerant person holds antagonistic views towards intolerance, and hence is intolerant of it. The tolerant individual would then be by definition intolerant of intolerance.
Philosopher Karl Popper defined the paradox in 1945 in The Open Society and Its Enemies Vol. 1. 
“Less well known is the paradox of tolerance: Unlimited tolerance must lead to the disappearance of tolerance. 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.”
He concluded that we are warranted in refusing to tolerate intolerance:
“We should therefore claim, in the name of tolerance, the right not to tolerate the intolerant.”
In 1971, philosopher John Rawls concludes in A Theory of Justice  that a just society must tolerate the intolerant, for otherwise, the society would then itself be intolerant, and thus unjust. However, Rawls also insists, like Popper, that society has a reasonable right of self-preservation that supersedes the principle of tolerance:
“While an intolerant sect does not itself have title to complain of intolerance, its freedom should be restricted only when the tolerant sincerely and with reason believe that their own security and that of the institutions of liberty are in danger.”
In a 1997 work, Michael Walzer  asked “Should we tolerate the intolerant?” He notes that most minority religious groups who are the beneficiaries of tolerance are themselves intolerant, at least in some respects. In a tolerant regime, such people may learn to tolerate, or at least to behave “as if they possessed this virtue”.
- Popper, Karl, The Open Society and Its Enemies, volume 1, The Spell of Plato, 1945 (Routledge, United Kingdom); ISBN 0-415-29063-5 978-0-691-15813-6 (1 volume 2013 Princeton ed.)
- Rawls, John, (1971). “A Theory of Justice”: 220
- Walzer, Michael, On Toleration, (New Haven: Yale University Press 1997) pp. 80-81 ISBN 0-300-07600-2
One response to “The paradox of tolerance”
Another quote from Karl Popper on tolerance.