The first prime minister of the state of Israel, David Ben-Gurion, was said to have once written to 400 scholars and asked them: "Who is a Jew and what is a Jew?" He received, the story continues, 400 different answers.

The decision by the Supreme Court to insist that the over-subscribed Jewish Free School (JFS) in London must use religious rather racial definitions to select its pupils has reignited an ancient debate about what constitutes the nature of Jewishness. Different people locate the answer in different places somewhere inside a triangle which is bounded by faith, ethnicity and culture – with identity, religious practice and the politics of Zionism each pulling the definition in contrary directions.

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Some people – and not just Orthodox Jews – are alarmed by the court’s decision. They see it as the state interfering in the right of a religion to delineate itself. The oldest definition of Jewish identity comes from halakha – the collective body of Jewish religious law, including the Bible, the Talmud and centuries of rabbinic law – which says that a Jew is anyone born to a Jewish mother or who undergoes a rigorous conversion process.

This means that the child of a militant atheist is deemed to be Jewish if his mother was. The Orthodox see religion and ethnicity so closely intertwined that they object vehemently to the idea the law can force them to concoct a test to determine Jewishness based on whether someone goes to synagogue, observes dietary laws and the like. “Having a ham sandwich on the afternoon of Yom Kippur doesn’t make you less Jewish,” the chairman of the Rabbinical Council of the United Synagogue, Rabbi Yitzchak Schochet, said recently.

To do that would create two classes of Jews – one observant, the other not. “This is a notion that is utterly false and to our thinking repugnant,” wrote Rabbi Dr Naftali Brawer, of one of the largest and youngest Jewish United Synagogue congregations in the country at Borehamwood yesterday in response to the Supreme Court decision. “A Jew is a Jew regardless of religious commitment and practice... We were not prepared to give even the impression that some Jews were more Jewish than others.”

The ruling “speaks directly to the right of the state to intervene in how a religion operates,” according to Stephen Pollard, editor of the Jewish Chronicle newspaper here. It is not for a secular authority to challenge the Chief Rabbi on what constitutes Jewishness, the argument goes.

And the Orthodox are supported on this by other faiths who fear increased conflict between secular laws and conservative interpretations of religious principles – citing the Christian registrar who this week lost her appeal against being fired for refusing to carry out gay civil partnership ceremonies, or the government’s refusal to allow Roman Catholic adoption agencies to refuse to place children with gay couples.

The last time the state attempted such definitions, they point out, was in Nazi Germany when the Nuremberg Laws classified people as Jews if they had three Jewish grandparents – and insisted they remained Jews if they became non-practicing, married outside the religion, or converted to Christianity. It also persecuted individuals with just a single Jewish grandparent, labelling them a Mischling, or ‘mongrel’.

The irony is that such ethnic definition is one which the Orthodox authorities – known in the UK as the United Synagogue, led by the Office of the Chief Rabbi – pretty much share. But not all Jews do. Those from the Reform and Liberal traditions consider someone Jewish if either parent was and the child has been brought up in the faith.

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Jewish identity is “primarily about thought and deed, rather than biology,” said Rabbi Danny Rich, chief executive of Liberal Judaism. In this week’s Supreme Court test case the father was a practicing Jew and the mother had converted to Judaism – but at a non-Orthodox synagogue. Rabbi Rich condemned the admissions policy of the JFS as “politically motivated”.

The current row once again exposes the level of sectarianism which causes bitter divisions in Britain’s community of 300,000 Jews. The Orthodox, who routinely declare the Conservative, Reform and Liberal movements “outside of Torah and outside of Judaism”, in effect brands 40 percent of Britain’s Jewish community as not proper Jews. Anyone who therefore converts in one of their synagogues is therefore deemed not to have converted at all.

So much so that the JFS would not even give a place to the daughter of its Head of English who converted to Orthodoxy in Israel but who then in an Orthodox ceremony in New York married a man named Cohen, a family which under Orthodox tradition is not allowed to marry converts.

Such behaviour makes a nonsense of modern notions of plurality and democracy, according to Rabbi Julia Neuberger, Liberal Judaism’s president of in Britain, who wants to radically redefine Jewish identity. “Just because the Nazis were obsessed with whether we had one Jewish grandparent or not doesn’t mean that we should be,” she was recently quoted as saying. “It would be an ‘up yours’ to the Nazis to be more accepting of converts. My view is that we should be incredibly grateful that anybody wants to be Jewish.”

Others go even further. Jews have never been genetically or otherwise ‘a people’, argues the historian Shlomo Sand in his new best-seller The Invention of the Jewish People. The ancestors of most Western Jews were medieval converts to Judaism, he suggests, arguing, highly controversially that Jews therefore have no exclusive right to modern-day Israel.

This flies in the face of most received wisdom that the Jews are indeed, inter alia, a race – which is why, at a recent roundtable for the Jewish Chronicle, its literary editor Gerald Jacobs commented people routinely say that someone “looks Jewish.” It is also why, thanks to historically low rates of intermarriage, Ashkenazi Jews (who make up 80 percent of all Jews) are one of the most coherent genetic groups that exist.

But identity transcends both race and religion. “I do none of those things which an Orthodox Jew would say are the things that define you as being Jewish,” said the writer Howard Jacobson at the same roundtable. There is a celebrated Talmudic story of a would-be convert who said to a famous rabbi: “I want to become a Jew, teach me the Torah while I stand on one foot”. He received the answer: “What is hateful to you, do not do to your neighbour. The rest is commentary. Go and learn.”

Jacobson locates his Jewishness in that story, in the love of commentary. “It doesn’t matter what you are commentating on,” he said. “It is the love of “go learn”. That is what it is for me to be Jewish.”

There are even those who locate the essence of being Jewish in being a community of suffering. The philosopher Sartre did that. Writing in 1948 he said that Jewish identity “is neither national nor international, neither religious nor ethnic, nor political: it is a quasi-historical community” and argued that the Jewish experience of anti-Semitism preserves – even creates – the sense of Jewish community. “It is the anti-Semite who creates the Jew,” he wrote.

Many Jews agree. “Our enemies have shaped Jewish life,” said the journalist and broadcaster Jonathan Freedland at the roundtable. “People say: ‘Because they hate us, because we survived to defy Hitler, those are the reasons why you have got to stay Jewish’. A number of people’s Jewish identity is formed out of persecution stories”. Yet that will not do either. Israel’s ruthless treatment of the Palestinians gives the lie to the self-definition of powerlessness and the threat of annihilation.

“We are neither a race nor a religion. We are something like a civilisation,” said Freedland. Or a family, says the American tv rabbi, Shmuley Boteach.

“It seems to me that the logical outcome of this discussion could be that there is no such thing as a Jew,” concluded Gerald Jacobs wryly.

Perhaps we should leave the last word with Sigmund Freud. “Since I don’t believe in any religion whatsoever, including Judaism,” he once said, “and since I despise all forms of nationalism, including Zionism, it may be asked what, then, is left of me that remains Jewish - to which I would reply, a very great deal and probably its very essence.”