Which came first, antisemitism or the Jew? Why not ask the boxer?

In each case, we have a presumption of antisemitism, verging on anticipation, expectation, or invocation. Far from all the fine work and policies in so many enlightened countries prohibiting "hate" crime, there is a counterpoint tending towards something very like the opposite of a prohibition. Underlying all this is a premise (so obvious that it is practically invisible) that the philosopher René Descartes might have expressed in this way: "I think, therefore I am a Jew." In other words, I know what it means to be Jewish. What can be called the logic, or perhaps the metaphysics of Jewish identity, was once brilliantly - if controversially - clarified by the great, 20th-century French thinker, Jean-Paul Sartre.

He was already so ugly that there was nothing his adversaries could do to him that hadn’t already been done

Sartre is probably best-known now for his resonant one-liner (from his play No Exit): "Hell is other people". Being and Nothingness - the immense "essay on phenomenological ontology" which he started in Stalag XIID in 1940 and completed at the Café de Flore in Occupied Paris - can be seen as a meditation on the inevitability of conflict.

Sartre often referred to himself as "the boxer", and boxing provides his recurrent image of human relations in general. His long-time companion, Simone de Beauvoir, citing Hegel, put it more emphatically in the epigraph to her novel, She Came to Stay. She wrote: "Each of us seeks the death of the other."

It was hardly surprising that they should have taken this view. It must have seemed like a description of everyday life. People they knew were being arrested and killed. Others were collaborating. Thousands of Jews were being routinely rounded up and shipped off to die in the camps. This was the high tide of the Holocaust. The Gestapo could knock at any time. It is odd then that, as Sartre put it in an essay written some time later: "We were never so free as under the German Occupation."

Sartre saw the individual broadly on the model of his own escape from the Stalag, in terms of liberation from confinement. The sense of freedom depends on a corresponding experience of opposition, the threat of domination. While the Nazis were intent on "stealing the future", Sartre defined his brand of existentialism in terms of the annihilation of history: the past (any idea of a genealogy, for example) was nothing, the self-defining "project" everything. There was one obvious choice at the time: you could choose to be a collaborator or a resister - on the side of the fascists or against them.

By the same token, any notion of self-identity was precarious and unstable, balanced on a tightrope. The Nazis wanted to reduce people to the status of things or objects, to be inspected and labelled. They could be measured. They had skin of a certain pigmentation. For Sartre, the human being was no-thing.

When he wanted to criticise his intellectual sparring partner, Albert Camus's attitude towards another man, he said that you "speak of him as though he were a soup tureen or a mandolin". Things have a clear-cut function, a purpose or meaning. Humans don't: they have to invent them.

Sartre was bound to come back to the concept of Jewishness after the war. Not only were Jews targeted by the Reich, objectified, scapegoated, excluded, exterminated but, conversely, the very assertion of Jewishness seemed like a mistake. In Réflexions sur la question juive, his 1946 work, Sartre argued that if Jews did not exist, antisemites would have to invent them. They made a weird, twisted sense of the world, and explained, in a magical way, why everything had gone so wrong with it. But - and this is the crucial insight that seems to me to explain why the guy in the Manhattan restaurant seemed to want Brits to hate Jews - it then also follows that Jews have need of antisemites. Why? Why on earth would you actually want to be hated?

Let me turn this around for a moment and relate another New York experience. New York, as someone once said, is the capital of a country that does not exist. Most of the time it seems exactly what it is: an offshore island (or archipelago). But for one day a year it really feels like an integral part of the United States - on July 4, the US Independence Day. New York joins forces with Americans at large by re-affirming their ancient hostility to colonial Britain. When I joined in the celebrations last year - on a boat on the Hudson River - I wasn't content with just watching the fireworks. I wore a Union Jack T-shirt. I was praying for somebody to pick a fight with me; to re-enact the original conflict and thus give me a sense of really being British. Alas, everyone I met was unfailingly polite and pretended not to notice the provocation. I suppose in some sense I was yelling out: "Why do all Americans hate the Brits?" I was actually disappointed that no one even came close to chucking me in the river.

Jews, Sartre argued, have no real genetic or historical or cultural foundation. They manifestly don't have a common faith (maybe they should, some would say, but in reality they don't). They are a loose collage of myths and rituals ("quasi-history"). Their sense of solidarity arises only in the face of antisemitism. Contrary to all the fascist propaganda that went into overdrive in the 1930s and the Second World War, there is nothing inherent in all the disparate people who might lay claim to a Jewish identity, that could conceivably give rise to or justify the hostility of the antisemite. Antisemitism is irrational. But Jewishness, likewise, is (to use the word popularised by Camus) absurd.

And here we come to the core of the philosophical argument that Sartre boldly advanced (and that still sends shivers through anyone of a nervous disposition).

Jewishness depends for its very existence on antisemitism. Antisemitism is the origin of Jewishness, not the other way around. Only at the very moment when that identity is threatened with annihilation can it be fully affirmed. Which explains the Sacha Baron Cohen line (which is, and is not, a joke): I need to remind myself of Nazism (possibly by dressing up in Nazi uniform) in order to feel good about being a Jew. It is the exact corollary of the Beauvoir epigraph: providing that the other out there wants to do me down (or in), then I exist. The more that the other seeks my death, the more alive I am.

And there is no escape from this argument by erasing "I" and rewriting it as "we". It is often said that Sartre's thinking was inflected with Marxism. And it is true that Groucho remains at the core of existential thinking: "I don't want to belong to any club that would have me as a member."

The point about being a Jew is that you are signing up to a club. You are a supporter of a team, a fan, a follower. Maybe it's a little like joining a fairly large Facebook group. So are we friends or aren't we? That is the question that comes back time and time again in the writings of (to give two arbitrary examples that spring to mind) Woody Allen and Philip Roth ("now that is a real 'Jew'! or is it?"). To be or not to be?

Certainly it seems to make a nonsense - an "absurdity" even - of any notion of a "Jewish state" (do we want a state governed by West Ham United, for example, since they have just been elevated to the Premiership, or one run by Spurs? ('Arry for president?) Or rather the Jewish state is to be fundamentally undecided about whether or not there can be such a thing as Jewishness in the first place. To come back to Sartre's original insight: Jewishness is a thing - a construct, an architecture of dreams - but I am no-thing.

To some extent Sartre's purely philosophical point chimes in with more historically orientated but sceptical works such as - most recently - Shlomo Sand's The Invention of the Jewish People. Sartre would add that Jews have to be perpetually reinvented. But for anyone who is outraged by the enunciation of the unthinkable, the consolation is that exactly similar thinking applies to Muslims. And even to the US itself, which loves a good enemy. So much so that they will happily take on someone else's.

Sartre discovered his own loved/hated antagonist in Albert Camus. Camus was the handsome one - a movie star among philosophers - while he, Sartre, looked like something hanging off the outside of Notre Dame. His main advantage when it came to boxing was that he was already so ugly that there was nothing his adversaries could do to him that hadn't already been done.

But here is the ultimate irony. Sartre found himself identifying with the Jews. He was a wannabe Jew. Or a fellow traveller. He reminds me a little of the dentist in Seinfeld who converted to Judaism in order to be able to tell Jewish jokes.

He identified with the proletariat, too, and (inverting the fascist propaganda that all Jews were capitalist fat cats involved in some kind of global conspiracy) saw Jews and workers as suffering similar kinds of persecution and exclusion. There were moments in his work and notably his later interviews (notably with the young Benny Lévy), in which he seemed to swing around to a more classically Jewish view of Jewishness and allow scope for notions of tradition and legacy.

If I am going to be brutally honest here, this was his least compelling period. It sounds like a boxer in his punch-drunk phase, reeling on the ropes. But if we go back to his epic mid-period in which he took on all-comers and knocked them flat, we find a bracing idea: the Jew is no one in particular; the Jew is anyone. That fabulous identity is a thing of smoke and mirrors. But then we are all Jews, groping after some meaning and reason and history that ultimately eludes us. Jewishness is the philosophical state par excellence.

Andy Martin is the author of "The Boxer and the Goalkeeper: Sartre vs Camus", published on June 6 (Simon and Schuster)