I disapprove of what you say, but I will defend to the death your right to say it.
This sentiment is attributed to Voltaire though he never actually said it (even in French). The nearest quotes are: "Think for yourselves and let others enjoy the privilege to do so too." and "Monsieur l'abbé, I detest what you write, but I would give my life to make it possible for you to continue to write."
It is a defence put forward by those who give room to unpopular or discredited views. Recently a film was shown at Cambridge University which propagated the discredited view that HIV does not cause AIDS. "Defence of free speech" screamed the organizers. Similarly, the BBC decided that the British National Party should be given time of British TV since many people voted for them at the European elections. It should be clear to everyone that the support for the BNP echoes their dislike of the European Union, but in no way endorses their racist policies.
So what position should rational people take of the free speech defence?
We do not allow racists to incite people to kill or attack black people, though we allow much more leeway to Islamists who encourage violence against Christians and Jews. We prosecute people who assert that the Bible opposes homosexual acts, yet we suspend police officers who decline to guard Gay Pride marches in which the marchers engage in acts of public indecency. Often our attitudes to censorship are colored by the political views of the government.
There are many instances where unpopular views have been censored by the church, or by government. Michael Servetus (whose 398th anniversary is on Tuesday), Galileo, Aung San Suu Kyi (incidentally did you know that it was the policy of the British Government to still call the country 'Burma' rather than 'Myanmar' because that is what the Democracy Movement there prefers?), William Tyndale and Gerry Adams are all people whose opinions were censored. Servetus and Tyndale paid the ultimate price for their opinions and so did Leon Trotsky. Censorship is surely preferable to assassination, and many would defend the silencing of Islamist preachers who have been the subject of 'control orders' by the British Government.
The libertarian view would be that we are grown ups and can decide for ourselves what we want to believe without the nanny state shutting people up. But what about those who are not grown up? Even libertarians will often endorse suppressing material aimed at children. Most people would want to restrict children's access to pornography and many would do the same for ultra-violent movies and computer games. Viewing child-pornography is regarded as a crime in many countries, principally because somewhere a child has been abused to produce the stuff. But how would the authorities regard Pixar-like cartoons of child pornography where every image was the figment of a warped imagination yet no child was harmed in its manufacture? Would it still be banned because of its tendency to incite?
There was a time when censorship of anything with a sexual content was routine. Bedroom scenes in movies were only passed by the censor if the actors kept one foot on the floor and even married couples always slept in separate beds. In Islamic countries similar standards are still applied. Yet we have become blase about watching advertisements with scantily-clad women (and men) much in evidence. The other day I visited the newly-restored Boscombe Pier. It sported photographs of the town at the beginning of the twentieth century. The clothes that women were required to wear when bathing at that time would astound viewers of Baywatch.
Cynics say that there is no such thing as bad publicity - getting your name in the papers is everything. Therefore when defenders of free speech publicise an unpopular view, intending to expose wrong-thinking to criticism, the 'wrong-thinkers' are delighted because some people will come out of the woodwork to agree with them and they are often able to misconstrue the public's response to them by lies and distortions. Benevolent liberals offer platforms in the interests of balance, but perhaps they would do better to be more judgemental. Discrimination has got a bad name, but we revere people with discriminating taste. Jesus said "Judge not that ye be not judged" but that text is usually taken out of context. We should certainly not be prejudicial, but elsewhere the Bible tells us to "test everything" and to "evaluate all things" and "choose between good and evil".
One of the great problems of denying free speech is in ensuring that you are not carried along by the popular view, just because it is the popular view and one endorsed by the the great organs of the press. I have made a study of scientific fraud and would no more agree with a point of view because "scientists have claimed..." any more that I would is "Barak Obama said..." or "Sarah Palin said...".
I admit to holding some unorthodox views myself. For example, I still think that there is a place for chlorambucil in the treatment of CLL. But I also have yet to see an adequate defence by Darwinists of how evolution escapes the rules of the Second Law of Thermodynamics or how they might come to terms with irreducible complexity. I find myself unconvinced by the idea anthropomorphic global warming. I am however convinced that the earth is not flat, nor is it travelling through space on the backs of four elephants supported by a giant turtle.
In science everything is contingent. Newton was correct until Einstein pointed out the exceptions. Our knowledge of the Universe is incomplete and science can never explain beauty, love, altruism, kindness, faith, harmony and self-sacrifice in anything other than descriptive terms. Stephen J Gould talked about different Magisteria; realms where science and religion held separate sway. I think he was right.
I am not sure that censorship is wrong in every circumstance.