The doctrine of toleration requires a positive as well as a negative statement. It is not only wrong to burn a man on account of his creed, but it is right to encourage the open avowal and defence of every opinion sincerely maintained. Every man who says frankly and fully what he thinks is so far doing a public service. We should be grateful to him for attacking most unsparingly our most cherished opinions.
Leslie Stephen, ‘The Suppression of Poisonous Opinions’, The Nineteenth Century, vol. 13, no. 73 (April, 1883), p. 665
If you think it would serve utility to ‘withdraw tolerance’ from such-and-such dangerous opinions, you’d better think through all the consequences. Your effort might be an ineffective gesture; in which case, whatever you might accomplish, you will not do away with the danger. Or it might be not so ineffective. To the extent that you succeed in withdrawing toleration from your enemy, to that extent you deprive him of his incentive to tolerate you. If toleration is withdrawn in all directions, are you sure the opinions that enhance utility will be better off? When we no longer renounce the argumentum ad baculum, are you sure it will be you that carries the biggest stick?
David Lewis, ‘Mill and Milquetoast’, Australasian Journal of Philosophy, vol. 67, no. 2 (June, 1989), p. 171