There are two arguments in favour of absolute freedom of speech: the principled argument, and the pragmatic one.
The principled argument is that if, like John Milton, we are to demand “the liberty to know, to utter, and to argue freely according to conscience, above all liberties,” then it is obvious that any limitations on speech begin to erode that liberty. To be able to set such limits on speech, then one must be in a position of authority and power. Even disregarding the anarchist stance that any authority which cannot bear a strong burden of proof is illegitimate, we are still left with the fact that anybody in power is, above all else, guided by a need to preserve that power. Thus, if they are willing to limit what you can say on seemingly reasonable grounds - for example, hate speech - then it will not be too long before they extend these limits.
If we are to try and censure that which is hateful or offensive, then an obvious question arises: who is left to decide what is hateful and offensive? What safeguards do we have against these definitions encroaching too far? Quite simply, we have none, as the existence of such “crimes” as treason and blasphemy demonstrate. It is not too hard to imagine, or indeed to see in the form of today’s “anti-terror” legislation, the very notion of dissent against those with power becoming a crime. As George Washington said, “if the freedom of speech is taken away then dumb and silent we may be led, like sheep to the slaughter.”
The most elegant summation of the principle of absolute free speech was made by Charles Bradlaugh:
Without free speech no search for truth is possible… no discovery of truth is useful… Better a thousandfold abuse of free speech than denial of free speech. The abuse dies in a day, but the denial slays the life of the people, and entombs the hope of the race.
The more pragmatic argument for absolute freedom of speech is quite simple: open and honest debate from all sides is a far more effective way of promoting good ideas and destroying bad ideas than censorship and oppression.
Too often, those who stand in opposition to bad ideas - be it fascism, racism, religious extremism, or anything else - talk about the need to deny such groups the “oxygen of publicity.” History and experience have shown, however, that publicity is only “oxygen” for good ideas that work to the benefit of people and society. To bad ideas, those that work to the detriment of liberty and progress, public exposure serves only as “poison.” They thrive underground, benefiting from their status as “martyrs” to censorship and “totalitarianism,” attracting to their cause so many who - following a human instinct to support that which is censored - would not be on their side if they truly knew the sordid details of what they are supporting.
In a world with truly free speech, however, you will encounter opinions you deem offensive to your view of the world. Because, as Salman Rushdie rightly notes, “without the freedom to offend, it [freedom of expression] ceases to exist.” Upon encountering such offence, it is your duty - and nobody elses - to oppose it; not with censorship, but with words of your own.
Any opinion that cannot withstand dissent and dissection is not worth hearing and will quickly be destroyed by reason. This is a basic principle of the Enlightenment, and an invisible regulator of discussion that makes censorship unnessecary and harmful.
Noam Chomsky has stated that “if you do not believe in freedom of speech for those you despise, then you don’t believe in it at all.” This is demonstrably true, and it is the reason why those who truly believe in free speech and freedom of conscience should repeat - even to the vilest enemy who wishes them dead - Voltaire’s maxim that “I may disagree with what you have to say, but I shall defend, to the death, your right to say it.”