No civilised society supports absolute freedom of speech: as the great American jurist Oliver Wendell Holmes argued in 1919: “The most stringent protection … would not protect a man in falsely shouting fire in a theatre, and causing a panic.”

Instead there is a fluid, rancorous, necessarily insoluble argument in every democratic system about where the border should lie. Libel, slander, false advertising, incitement to violence, pornography, the leaking of official secrets: these and other forms of expression are subject to varying degrees of restriction at different times.

UC Berkeley cancels 'alt-right' speaker Milo Yiannopoulos as thousands protest Read more

It is in this context that one should interpret the cancellation last Wednesday of the Milo Yiannopoulos event at the University of California, Berkeley. This was to have been the final date of the rightwing controversialist’s charmingly named Dangerous Faggot Tour of US campuses. But what started as a legitimate protest against Yiannopoulos fast degenerated into flames, violence and Black Bloc mayhem, forcing the talk to be cancelled.

Talk about an own goal. It should be crashingly obvious to all but the most tenacious squatters on the moral high ground that, far from silencing Yiannopoulos, such actions turbo-charge his fame. In November he was stopped at the eleventh hour from speaking at the Simon Langton grammar school for boys in Canterbury. The Chicago Review of Books has announced a year-long boycott of Simon & Schuster titles in defiance of the publisher’s book deal with Yiannopoulos.

Not since the Sex Pistols were banned from venues across the land has prohibition been so counterproductive. In the space of a few months Yiannopoulos has risen from minor notoriety as an editor at Breitbart known for his hostility to feminism, Islam and liberals to the rarefied heights of those who are routinely referred to by their first name alone: Beyoncé, Boris, Madonna, Oprah … and now, ludicrously, Milo.

Donald J. Trump (@realDonaldTrump) If U.C. Berkeley does not allow free speech and practices violence on innocent people with a different point of view - NO FEDERAL FUNDS?

Last week’s riots even prompted a tweet from President Trump, threatening Berkeley’s federal funding. As a pathological self-publicist, what more could Yiannopoulos really ask for? I hope all the balaclava-wearing rioters who have just driven up this unlovely prankster’s share price to an all-time high are pleased with themselves. Nice one, guys.

Yet this incident is a test case in a much greater debate. Full disclosure: I started my career as a researcher at the magnificent Index on Censorship, an experience that strengthened my instinct that free speech should prevail except in the most extraordinary circumstances. The onus should always be upon those who would curtail free expression to prove beyond reasonable doubt that restriction is justified.

In two respects, what happened to Yiannopoulos last week is a warning. The first is specific. In the era of Brexit and Trump, the alt-right – the loose-knit network that stretches from parts of Ukip to neo-Nazis and white supremacists – has found its voice amplified and (sad to say) heeded. It has one of its own, Stephen Bannon, at the right hand of the president, acting as his ideological ventriloquist. By all means, deplore these bigoted populists. But ignore them at your peril.

Facebook Twitter Pinterest ‘You may well despise Milo Yiannopoulos.’ Photograph: Jeremy Papasso/AP

It is a commonplace argument that even to confront the alt-right is to confer upon them a legitimacy they do not deserve. Respectfully, that moment is long past. Look at the team in the White House. Consider the ugly centrality of immigration to all political discourse in Britain during and since last year’s referendum. Talk all you like about “no platform”. Right now, this lot own the platform.

No: the way to beat the alt-right is to take them on, repeatedly, in every conceivable setting. Reliably aggressive on initial contact, they are often the most sensitive snowflakes you could imagine when cross-examined. In person, in print, on social media, even in court, these fact-averse charlatans are vulnerable to challenge. If there is a core lesson in Denial, the new film dramatising David Irving’s disastrous libel case against the historian Deborah Lipstadt, it is that the forensic demolition of a Holocaust denier is the most effective form of humiliation (certainly more damaging to Irving’s reputation than his subsequent imprisonment in Austria). For a masterclass in how much better it is to bombard your opponents with questions than bricks, watch Cathy Newman’s demolition of Yiannopoulos on Channel 4 News last November.

Second, there is a broader reason why the Berkeley riot should give us pause for thought. A dangerous complacency is entering contemporary thinking about free expression – as though it were a fixation of the past, a right that has served its purpose, and may now be safely curtailed in the interests of social cohesion or a particular definition of equality. We live in an age in which cartoonists are massacred, in which publishers and theatres increasingly shy away from provocative material, in which sensibilities are protected from outrage more rigorously than speech is protected from suppression.

In a pluralist society, the line of least resistance is to shield citizens from offence. The problem is that everyone is offended by something, or by many things. Those of us who still believe in Enlightenment principles find much that the religious say extraordinarily offensive, as well as ridiculous. But it would be absurd to curtail their freedom of worship on such grounds. A democracy in which offence was outlawed would not be worthy of the name. It would be a wasteland of stultified silence.

Milo Yiannopoulos peddles hate. It’s not censorship to refuse to publish it | Sam Sedgman Read more

In the US, the founding fathers assigned supreme value to free speech – and enshrined it in the first amendment – precisely because they understood how important it was in the battle against tyranny and the protection of pluralism. The more diverse a society becomes, the more it needs a free exchange of ideas – not least to protect minorities from the megaphone of the majority. The fact that Britain is now a multifaith, multi-ethnic society is an argument for more freedom, not less.

The inconvenience suffered by Yiannopoulos and his audience last week will soon be forgotten. But the principles at stake should not be. The boundary that separates free speech from prohibited expression will always be mutable. What matters is where the presumption lies; and in a decent society, the default impulse should always be to defend such freedoms. One need only observe Trump’s incremental withdrawal of rights – the migration ban, the threats to the press – to see how perilous it is to take them for granted.

You may well despise Yiannopoulos. But the right to free speech is meaningless unless it is extended to those with whom one profoundly disagrees. Those who argue otherwise are simply doing the far right’s work for them.