Every publishing and communications medium, from the printing press to the broadcast media to the internet, has suffered from attempts to censor and control its content. In previous eras, content was claimed to pose a threat to public or religious morals. Today, when there is less of a consensus over religion and morality, censorship is justified by appeals to multicultural tolerance and the protection of children. Hate speech and paedophilia are now the most talked-about evils online, joined recently by terrorism.

But the shift in the emphasis of the censor's concerns does not make censorship any less dangerous. Today's censors share with their predecessors the patronising belief that thought is inseparable from action – that consuming objectionable material will either impel certain people to do objectionable things, or damage them irreparably.

This is why the internet – which allows for the common indulgence of potentially any interest between any number of like-minded individuals, anywhere in the world – has elicited such widespread attempts at censorship. The tragedy is that so many internet users are blissfully unaware that their medium is censored, because the censorship slips by in sheep's clothing.

To be sure, there are still old-fashioned legal moves to enforce internet censorship. The US Congress is involved in ongoing attempts to pass laws criminalising certain types of internet content. Having failed to pass the Communications Decency Act in 1997, Congress is this year attempting to introduce the Child Online Protection Act.

But crucial though it is to fight and win the case for free speech in the traditional arena of law, there are more insidious threats to free speech online. Since the internet poses several obstacles to straightforward censorship – it is a decentralised, international medium accessible by all and traceable to few – unaccountable censors and institutionalised self-censorship are the order of the day.

Given the difficulty of attributing responsibility for internet content to a specific author, recriminations for objectionable content tend to fall at the feet of internet service providers (ISPs), the companies who make it technically possible to access content. Unaccountable institutions such as the Internet Watch Foundation (IWF) in the UK and the Australian Broadcasting Authority (ABA) in Australia advise ISPs to remove objectionable material, and more often than not the ISPs comply.

Such censorship is often presented as democratic expression of the public will, on the grounds that the censors are independent from the state and compliance with their orders is voluntary. But when you consider that most "voluntary" requests by the IWF to remove content are complied with by ISPs – they wouldn't want bad press, after all – you can see why the Home Office and the police are so keen to endorse such an "independent" body. The IWF conveniently circumvents the need to justify censorship in a court of law.

In their quest to censor internet content via the back door, the authorities are also quick to champion blocking and filtering technologies such as Cyber Patrol and Surf Watch, which screen out certain kinds of content for those who elect to have it screened. The emphasis, again, is on the voluntary character of the censorship, as though its being voluntary is a blow for democracy.

But how is it an expression of the public will, for the decision-making processes of the internet user to be handed over to a third party? Let adults have to make individual decisions about what they're looking at, and go elsewhere if they don't like it – they're big enough.

Now the influence of censors is extending beyond the world wide web to other components of the internet. Online file-sharing software, such as Limewire, Aimster and iMesh, is the subject of new concerns about pornography. And since the terrorist attacks of 11 September 2001, governments and lobbyists have had a field day insisting that civil liberties should be forfeited to the maintenance of safety by the state.

Speaking on BBC Radio 4's Today programme on 28 September 2001, UK foreign secretary Jack Straw went as far as blaming the 11 September attacks upon "a two-dimensional view of civil liberties" on the part of those who champion online freedom, and claimed that such people "will now recognise they were naive in retrospect".

At the risk of disappointing Jack Straw, we recognise no such thing. Internet censorship can never be an act of public good. What is a public good is the continued existence of a communications medium for the universal exchange of ideas. In comparison with this, the existence of some material that appals us (which we don't have to look at), and the fact that we might want to supervise our children when they use the internet (heaven forbid), is a small price to pay.