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
[I]t is not the minds of heretics that are deteriorated most, by the ban placed on all inquiry which does not end in the orthodox conclusions. The greatest harm done is to those who are not heretics, and whose whole mental development is cramped, and their reason cowed, by the fear of heresy. Who can compute what the world loses in the multitude of promising intellects combined with timid characters, who dare not follow out any bold, vigorous, independent train of thought, lest it should land them in something which would admit of being considered irreligious or immoral? Among them we may occasionally see some man of deep conscientiousness, and subtle and refined understanding, who spends a life in sophisticating with an intellect which he cannot silence, and exhausts the resources of ingenuity in attempting to reconcile the promptings of his conscience and reason with orthodoxy, which yet he does not, perhaps, to the end succeed in doing. No one can be a great thinker who does not recognise, that as a thinker it is his first duty to follow his intellect to whatever conclusions it may lead.
John Stuart Mill, On Liberty, in The Collected Works of John Stuart Mill, Toronto, 1988, vol. 18, p. 242