Sartre: the philosopher of the 20th century by Bernard-Henri Lévy, trans. Andrew Brown

Sparks fly when today's French philosophical superstar confronts his great ancestor. David Coward watches one Left Bank idol grapple with another

When Jean-Paul Sartre died in 1980, 50,000 sorrowing faithful followed him to Montparnasse cemetery. The figure was well below the 2 million who gave Victor Hugo his send-off in 1885, but it dwarfed the turn-out for the passing of the 20th century's other literary giants or the structuralist warriors - Foucault, Althusser et al - who had unseated "the Pope of existentialism".

Although Sartre was genuinely mourned in some quarters (and vilified in others), he was not greatly missed. The season ticket to the front pages which he held for 40 years had expired. Without new provocations to stir controversy, his reputation continued to dwindle, swallowed up in clouds of postmodernism. Now, 20 years on, Bernard-Henri Lévy, philosopher and media star, offers a reassessment of the century's "total intellectual".

Philosopher, novelist, playwright, journalist, literary critic, political activist and thorn in the flesh of bourgeois everywhere, Sartre began in the Thirties with a radical theory of human freedom. There is no God and the universe goes about its business without reference to us: our existence has no point except itself. Whereas Camus concluded that suicide was therefore the only serious philosophical issue, Sartre believed that a life could be given meaning if it is committed (engagé) to a coherent course of action.

In his novel Nausea (1939), he suggested that creating a work of art is one way of giving a life sense. Sartrean engagement, developed in Being and Nothingness (1943), was used to justify resistance to the German occupier in The Flies, his play staged in 1943. But it was still, at this stage, a duty of the individual not the group. Huis Clos (1944) warned that "hell is other people", for we vie with them in constant competition for attention.

Unless we remain flexible in our responses, accept the consequences of our actions, and act in good faith with ourselves, we become "inauthentic", hypocritical "bastards" and create all the ills that society is heir to. It is hardly an endorsement of collective action.

Like most critics, Lévy finds most to admire in this early Sartre, but for rather different reasons. Extistentialism was not, as the title of Sartre's most famous lecture puts it, "a Humanism" at all. Humanists always set out to make the world better by eradicating the Old Adam who will be persuaded, by force if necessary, to mend his ways.

Initially, engagement was not authoritarian but personal, apolitical and only indirectly a viable social philosophy. If all men act in good faith, then no man will promote totalitarianism, fascism, colonialism, racism, fundamentalism, and so forth. It is in this lack of direct interest in collective problems that his philosophy was "anti-humanist", a recommendation in Lévy's vocabulary.

Had Sartre stopped there, all would have been well. But after the war he applied engagement increasingly to the collectivity: the group must be made to behave authentically. In the process, a philosophy which had been anti-humanist and anti-fascist now became illiberal.

All totalitarian bullies, argues Lévy, begin as idealists who wish to make the world a better place. They assume that community is humankind's natural state and that it is right to restore it. Having persuaded the people to accept these myths, they proceed to lie and deceive to improve the world, and use prison, torture and murder as the means of justifying an end whose coming is permanently postponed.

For Lévy, Sartre turned into just such a totalitarian activist. He defended some of the most despotic regimes in history, travelled the world looking for surrogate proletariats to defend, loathed American imperialism, approved of violence as a political tool, refused, unlike Koestler or Orwell, to see Soviet Russia as fascism in red trousers, and went into the Seventies needing to believe in Mao's Oriental Eden. Sartre was probably the "last Stalinist", yet he himself remarked that he had spent all his life fighting to create a society in which he would not have wanted to live.

For his pains he was called a peddler of filth, a jackal with a pen. His books were placed on the Catholic Index in 1947 and attempts were made on his life. He was accused of corrupting the nation's youth and he treated friends like Camus and Raymond Aron quite odiously.

Lévy pulls no punches (there are distinct sounds of an idol falling). Yet he also writes with affection, rescuing from Sartre's squalid private life his strange relationship with Simone de Beauvoir, which he calls "one of the most beautiful love stories of the 20th century".

As to why the early libertarian Sartre turned into an intellectual terrorist, he suggests a mixture of technical philosophical reasons (notably his failure to find answers to Hegel) and a basic human reaction: the lonely writer's sudden discovery of comradely solidarity in his POW camp in 1940. But his "barbarism" was also an effect of his disillusionment with literature. He came to regard it as a lie because it makes us take words for things and images for reality. Literature was his "neurosis" and, just as he abandoned philosophy, so with his childhood memoir, Words (1964), he finally put literature aside. It was only logical, therefore, that he should refuse the Nobel Prize in 1964, for in so doing he formally rejected what had made him a writer.

Lévy's estimate of Sartre - a personal philosophy of freedom and a public life lived increasingly with "obtuse stupidity" - is less earth-shattering than its intimidating bulk and distracting cleverness promise. Learning here is not worn lightly but brandished violently, as though to warn away the cissies for whom this book is not intended. The class is assumed to have read all the books and the lesson goes at the pace of the brightest. Showy asides on Heidegger, Althusser and Hegel, names flashily dropped, a tendency to write orally, as though for an earnest chat or a public meeting, and expressions like "originary historiality" suggest that Lévy needed a no-nonsense editor. On the positive side, in Andrew Brown, he was given a splendid translator.

David Coward's 'A History of French Literature' is published by Blackwell