Some linguists say he is as vital to their field as Albert Einstein is to physics. In 1957, he published his first book, Syntactic Structures, arguing that our ability to produce sentences is biological and not simply learned behaviour, as the 'structuralist' orthodoxy had it. Chomsky posited a 'universal grammar' that all people share; the task for linguists is to describe these inborn principles, rather than just the grammars of different languages.

Growing up in Philadelphia during the Depression, Chomsky thought about language and politics from an early age. His father, William, a Russian emigrant, was a respected Hebrew scholar and an expert on medieval grammar. Elsie, his mother, emigrated from Lithuania as an infant and taught at a Hebrew school. Chomsky says of the pre-war years: 'People were suffering. You could see it everywhere. But it was an intellectually and politically very lively time.'

At weekends he commuted to New York to help his disabled Trotskyite uncle run his newsstand business, and Chomsky absorbed the heated debates that took place there. He hung out in anarchist bookstores, read political pamphlets and chatted with European refugees. When Barcelona fell in 1939, the 10-year-old wrote an editorial for the school newspaper about the ascent of European fascism.

At 21, he married Carol Schatz, and by the mid-60s, his academic celebrity was well established. But as the US started bombing North Vietnam, he felt he had more urgent callings than pondering syntax. Activism consumed him. Chomsky shared a cell with Norman Mailer after participating in the march on the Pentagon in 1967.

Chomsky's wife returned to university to complete a doctorate in linguistics so she could support the family if Chomsky was jailed or lost his job with MIT - a major risk, as the majority of the school's budget was then coming from the Pentagon.

His first political book, 1969's American Power and the New Mandarins, collected his writings on Vietnam. Its arguments would form the cornerstone of his critique of subsequent American military interventions: the government's humanitarian rhetoric is merely a cover for its imperial ambitions, and liberal intellectuals help legitimise its atrocities.

After Cambodia fell to the Khmer Rouge in 1975, his hatred of American foreign policy led him to write sympathetically about Pol Pot. In 1978, Chomsky and Edward Herman published After the Cataclysm, in which they rationalised the Cambodian communists' lethal social engineering as an understandable remedy to the economic devastation caused by US attacks.

Nowadays, Chomsky is more tactful. His writings on the Khmer regime had a specific context, he insists - contrasting the Western media's treatment of crimes committed by the West and its proxies, and those perpetrated by enemy states.

In 1975, Indonesian president Suharto, a US ally, invaded East Timor and by some accounts killed more than 100,000 civilians. To Chomsky, 'It was our crime, we could do a lot about it, and there was silence and denial. By now, the facts are all there. If you choose to deny them it's a definite choice. It's like Holocaust denial.

'There's a literary genre now - Samantha Power is the leading figure - in which we denounce ourselves for our failure to respond to the crimes of others. But we either totally ignore or else flatly deny our own crimes.'

His call for Israel to become a bi-national state makes him a figure of hate for many Jews, who argue that a system in which Jewish people are a minority in an Arab-dominated country would be suicidal. 'I think the hostility would decline,' Chomsky says.

Even those who tolerate Chomsky's views on Israel are usually troubled by his association with neo-Nazis. In 1979, he signed a petition defending the right to free expression of French literature professor Robert Faurisson, who had been suspended from teaching for his Holocaust denial.

Chomsky wrote Faurisson's publishers an essay describing the literature professor as 'a relatively apolitical sort of liberal', and told them to use it for the campaign. When he learned that Faurisson was using the essay as a preface to his book Memoire en Defense (Memory in Defence), Chomsky changed his mind - but it had already gone to print. 'I tried to retract it and that I feel was a mistake,' he says. 'I knew that in France where there is total irrational hysteria among the educated classes, and also absolute hatred of freedom of speech, it would be grossly misinterpreted.'

For Chomsky, the media are an elaborate mechanism for state thought control: 'The Western intellectual community is dedicated to lying in support of state power.' So media consumers and journalists are dupes, incapable of independent thought? 'These are tendencies,' he says. 'There are some very decent people and fine journalists.'

In an era of obsessive public opinion polling, it can be argued that politicians are actually excessively attentive to the whims of popular opinion. But Chomsky downplays the influence of polls on politicians, adding 'one of the ways of protecting the leadership from the public is simply by not publishing the results of opinion studies. That's the norm.'

To Chomsky's mind, the US is a four-year dictatorship. 'The way our system works is that once every four years you have a choice between two candidates - both of whose positions you oppose - and after that, 'Shut up.''

Chomsky calls President Barack Obama 'dangerous'. 'He's a centre-right Democrat who insists that the US is an outlaw state, which must violate international law and resort to violence when it chooses.'

After September 11, Chomsky gained renewed popularity on the hard left. He was virtually alone among commentators in protesting against the American military action in Afghanistan, which he termed 'silent genocide'. His pamphlet-sized book of interviews, 9-11, sold hundreds of thousands of copies.

Yet columnist Christopher Hitchens, formerly a champion of Chomsky, charged him with 'losing the qualities that made him a great moral and political tutor in the years of the Indochina war'. Hitchens was disturbed by Chomsky's suggestion that president Bill Clinton's 1998 bombing of the Al-Shifa pharmaceutical plant in Sudan, in which a security guard was killed, was comparable to the attacks of September 11.

Chomsky survived a cancer operation a few years ago, but he's now in good health.

He's not afraid of his own mortality. 'As a child, I thought it was an indescribable horror. That's passed over the years. I feel that I'm already a decade beyond what life is supposed to be according to the holy texts.'

His assistant throws open his door; Chomsky is busy today - being photographed for his old primary school. He puts on the school cap and flashes his gap-toothed smile for the camera. And for a moment the professor with no ego or charisma looks rather pleased with himself.

Writer's notes

Name: Noam Chomsky

Age: 81

Born: Philadelphia, Pennsylvania

Lives: Cambridge, Massachusetts

Family: Married to Carol, a linguist, with two daughters, Diane and Aviva, and a son, Harry

Books include: Syntactic Structures (1957), American Power and the New Mandarins (1969), Language and Responsibility (1979), Manufacturing Consent (1988), Necessary Illusions (1989), Deterring Democracy (1992), Year 501 (1993), World Orders, Old and New (1997), Fateful Triangle (updated, 1999), The New Military Humanism (1999), Rogue States (2000), New Horizons in the Study of Language and Mind (2000), A New Generation Draws the Line (2001), 9-11 (2001), Hegemony or Survival (2003), Failed States (2006).

What the papers say: 'The appeal lies in the simple argument that underlies the convoluted prose. Capitalism, particularly American capitalism, is responsible for the world's problems. Resistance ... is inevitable. If the resistance is barbaric the barbarism is the fault of capitalism.' The Observer on Hegemony or Survival 'Chomsky argues that the situation is now so extreme that democracy is no longer functioning in America.' The Daily Telegraph on Failed States