In the late 1980s, the Italian President Francesco Cossiga described Antonio Negri as "a psychopath" who "poisoned the minds of an entire generation of Italy's youth". Negri has been accused of murdering Italy's former Prime Minister, Aldo Moro, and of being il grande vecchio - the grand old man - behind the Red Brigades, one of the most notorious terror groups to attack post-war Europe until al-Qa'ida. In prison he co-wrote an anti-globalisation bible, Empire. Now he's out, and he's heading to London. I am waiting patiently at the Institute of Contemporary Arts, in the shadow of Buckingham Palace, to have my mind poisoned.

"I'm afraid Antonio's gone Awol," his publicist explains, diffidently. And he's due to speak to a crowd of 200 at the ICA in just an hour. "We sent a taxi to pick him up and... well... he's not there. Apparently, this happens." I take this opportunity to have another go at finishing Empire. An unexpected bestseller, the book was written by Negri and an American friend and fellow-academic, Michael Hardt, in the late 1990s, and has now topped 40,000 sales. Thanks to its impossibly dense thickets of sociological prose, it has developed a reputation as the Finnegans Wake of political writing, a book much purchased but never finished.

Here is a typical Negri sentence, selected at random: "The analysis of real subsumption, when this is understood as investing not only the economic or only the cultural dimension of society but rather the social bios itself, and when it is attentive to the modalities of disciplinarity and/or control, disrupts the linear and totalitarian figure of capitalist development." After 400 pages of this, I feel like I have been raped by a dictionary of sociology.

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As I try for the thousandth time to figure out what the hell "biopower" is (and how it will motivate the "posse" to "replace the City of God with the City of Man"), I notice a man walking directly towards me in a neat suit. He doesn't smile. "I need wine," he says quickly, as he lights a cigarette. "White wine."

From the vantage point of the bar, I look at the philosopher-terrorist. Now 71, he is tanned and tall, with a slight stoop. Was this bland-looking man really the cause of so much rage? Negri first became notorious in the mid-1970s, when the Italian far left began to fracture. The Italian Communist Party decided to enter into a coalition government with the Christian Democrats. The shard of the far left that could not accept this "Grand Historic Compromise" grew sharper and bloodier, and began to advocate immediate revolution. Negri was the guru of the new movement for "permanent civil war" and "mass illegality".

Negri identified himself with the violent protesters who wore ski masks as a disguise. He wrote: "I live the life of the sniper, the deviant, the criminal, and the worker who doesn't show up for his job. Every time I put on my ski mask, I feel the warmth of the proletarian community around me... Every action of destruction and sabotage seems to me a manifestation of class solidarity. Nor does the eventual risk bother me: rather, it fills me with feverish excitement, like a man waiting for his lover. Nor does the pain of my adversary bother me."

He smiles as I hand over his wine. Michel Foucault famously said that Negri was imprisoned in 1979 by the Italian authorities "for being an intellectual", so I start the interview by trying to clear up a question that has been nagging at me. What crimes did Negri actually commit? He was jailed only for the nebulous offence of "leading a subversive organisation". His group Autonomia Operaia (Workers' Autonomy) committed 174 attacks against civilians and 206 robberies; so Antonio, in which of these acts did you participate?

He looks at me very closely, with mild displeasure. He says in a level voice: "I never made an attempt on anyone's life." Then, with a shrug, he says to his translator: "I was accused of having committed hold-ups." So, was that accusation accurate? He takes a long drag on his cigarette. "Stealing money, if it's necessary, I can understand." I wait for him to continue, but the sentence hangs there, like his fading smoke. Did you rob banks? "Brecht said that it's hard to know which is a greater crime, to found a bank or to rob one," he replies. More waiting, more smoke. He pushes his glasses on to the top of his head with his taut middle finger. "I agree with Brecht," he says, waving his hand as though to physically push me on to another question.

I am about to try once again, but we are interrupted by an angry woman. "Have you seen that?" she says to Negri, officiously. He turns to me, puzzled, as though this is part of the interview. It occurs to me with horror that she might be a relative of one of his victims. "Well? Have you?" she barks. Then I spot it: the "No Smoking" sign. He does not look at her but places his cigarette in the ashtray. Appeased, the ICA staff-member marches off. Negri picks up his cigarette, resumes smoking and nods at me to continue.

He obviously does not want to talk about the Seventies. He insists that he renounces and regrets nothing, but won't offer any justifications, either. Eventually, he says: "It was necessary to respond at the same level as the police." Here, at last, is something of an explanation. To be fair to Negri, his group was not protesting against a normal liberal, democratic state. By the time he became politically active, the Italian police, military and judiciary were riddled with far-right Mussolini nostalgics who - in concert with the CIA - used police-state tactics.

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They targeted Italy's broad-based left-wing movements - which still commanded 35 per cent of the vote - and, we now know, even launched terror attacks that were then pinned on the protesters.

But perhaps the roots of Negri's philosophy lie even further back. He was brought up in Padua in the 1930s, then one of the poorest and most priest-infested parts of Europe. His father, a worker for the local council, was a founder of the Italian Communist Party in 1921, and paid for it with his life. Local Fascists beat and humiliated the family, and, finally - when Antonio was two - they murdered his father. "They forced him to drink castor oil," Negri explains. "It was like drinking dirty motor oil. It causes blood poisoning. It just empties you out."

He lights another cigarette. "And so he died." I wonder how, given this personal hell, he can trivialise Fascism as readily as he does. In one of his recent books, Negri actually defines it as "all the forces that stand in the way of desire and seek to block its emergence and expression". Surely this is the politics of a 13-year-old? "You are part of this!" he says in sudden response. "You help to sell my books! So you are part of it. You cannot deny your responsibility."

My responsibility for what, exactly? He opens his mouth in response and lets out a low wheeze. It takes a few seconds for me to realise that this is in fact a laugh. I'm not sure what he is laughing at, but I join in nervously. Soon, we are chuckling quite openly, and I feel like I am lost in a fog. "More wine please," he says suddenly. "More wine."

When I return from the bar, it seems time to move on to his book Empire. Negri managed to flee to Paris in 1983, where he supped at the well of French intellectuals such as Jacques Derrida. When he voluntarily returned to custody in Italy in 1997, he did not leave behind the often incomprehensible academese of the Left Bank. Empire has been lauded as a rewriting of the Communist Manifesto for the 21st century, but it is written with all the lucid passion of a badly translated computer manual. George Monbiot, one of the most important intellectuals in the anti-globalisation movement, recently confessed: "There's a game I play with friends, which is to open that book at random, put your finger on a paragraph and see if you can work out what the hell it means." He adds, generously: "Negri has some important things to say. I just wish he had said them more economically."

But for all its unreadable gibber, it is still possible to pick out some continuities with Negri's earlier philosophy. He still describes himself as a communist, and still calls for a revolution. At moments, he seems to have emerged from cryogenic decades preserved in ice, like Austin Powers, oblivious to the fact that the world has kept on turning. Yet he does acknowledge some changes. Negri believes that we are all now living in a condition called "Empire". This isn't US imperialism; it is a dense, autonomous network of capitalist power that supersedes any nation-state. Empire exerts its control through "biopower" - a subtle form of manipulation that infests our brains and makes us internalise the values of capitalism. We live in a Truman Show world where everything has become fake, "subsumed by capital", turning us into mindless drones. Citizens of liberal democracies foolishly believe themselves to be free when, in fact, we are living in totalitarian "societies of control", a vast "social factory".

At least, I think that's what he is trying to tell me. He believes that Empire is a triumph, a positive development in the history of humanity. Just as Marx celebrated the victory of capitalism over feudalism, Negri says that the left should welcome Empire, because it clears away the old nationalisms and creates a space for opposing capitalism at a global level. Globalisation has broken the "infernal cage" of the nation-state, and we should rejoice.

Slowly, a Counter-Empire is emerging of disillusioned and resisting people - from South American revolutionaries to Islamic fundamentalists. Soon they will rise up and institute "global citizenship". Negri begins to laud, as he often does, "the communist and liberatory combatants of 20th-century revolutions", as if there was no contradiction between communism and liberation.

I try to think of a polite way to remind him of the fact that every communist revolution of the 20th century lead to tyranny and mass murder. And a nice way to say that communism was a betrayal of the democratic values of the left. I fail. I blurt it out. "These communist regimes are waiting for a historical revision. They may not be seen so negatively in the next century," he says, as though this was perfectly obvious.

Negri recently described the Soviet Union as "a society criss-crossed with extremely strong instances of creativity and freedom", which is more than he has ever said for any democracy. He even says that the Soviet Union fell because it was too successful. I point this out, and he replies: "Now you are talking about memory. Who controls memory? Faced with the weight of memory, one must be unreasonable! Reason amounts to eternal Cartesianism. The most beautiful thing is to think 'against', to think 'new'. Memory prevents revolt, rejection, invention, revolution."

He leans back as though he has brilliantly rebutted any critique of communism. So, is he seriously saying that we should never look at history, that the left should carry on as though communism was a great success, that we should not reconsider our values at all? "Look, a truth is a collective action on the part of persons who campaign together and who transform themselves," he says. This doesn't seem to correspond with what I asked him at all, but then I remember reading his essay, "In Praise of the Absence of Memory". I had assumed that I had misunderstood it, but now it becomes clear that he really does believe that it is better not to remember inconvenient facts, to "maintain a subjective point of view... because memory dulls the spirit. Memory is a prison". If you have to choose between history and communism, ditch history.

I can't take it any more. None of the world's real problems - from poverty to tyranny to climate change - are discussed in Negri's work, except to claim that the poor are "more alive", and the citizens of liberal democracies are living under the "real tyranny", and... oh, I give up. It's not just that this preacher of Empire has no clothes; he is living in an intellectual nudist colony. There are some important anti-globalisation writers, such as Monbiot and Joseph Stiglitz. But Negri is trying to keep alive a patient - Marxism - whose heart stopped beating long ago.

So, this is where revolutionary Marxism comes to die. It has been reduced to an obscure parlour game for ageing bourgeois nostalgics, played out a few feet from Buckingham Palace by an old terrorist who needs us to forget.