Multiculturalism has always been an embattled idea, but the battle has grown fiercer of late. In this, as in so many other things, it is terrorism that is setting the agenda, goading us and forcing us to respond ? terrorism, whose goal it is to turn the differences between us into divisions and then to use those divisions as justifications.

No question about it: it?s harder to celebrate the virtues of polyculture when even Belgian women are being persuaded by Belgians of North African descent to blow up themselves and other people.

Comedians, among others, have been trying to defuse ? wrong verb ? people?s fears by facing up to them: ?My name?s Shazia Mirza, or at least that?s what it says on my pilot?s license.? But it will take more than comedy to calm things down.

Britain, the most determinedly ?multiculturist? of European nations, is at the heart of the debate. According to some opinion polls, the British people avowed their continued support for multiculturalism even in the immediate aftermath of the July 7 bombings. Many commentators, however, have been less affirmative.

David Goodhart, editor of Prospect magazine, asks the old philosophical question, ?Who is my brother?? and suggests that an overly diverse society may become an unsustainable one. Britain?s first black archbishop, the Rt. Rev. John Sentamu, accuses multiculturalism of being bad for English national identity. And the British government has announced that new citizens will have to pass a ?Britishness test? from now on. A passport will be a kind of driver?s licence proving that you?ve learned the new rules of the nationalist road.

At the other end of the spectrum, Karen Chouhan of the 1990 Trust, a ?black-led? human-rights organization, insists that ?we need to move forward with a serious debate about how far we have to go in tackling race discrimination in every corner of society, not move it back by forcing everyone to be more (white) British?.

And Bhikhu Parekh redefines multiculturalism as the belief that ?no culture is perfect or represents the best life, and that it can therefore benefit from a critical dialogue with other cultures... Britain is and should remain a vibrant and democratic multicultural society that must combine respect for diversity with shared common values?.

It?s impossible for someone like myself, whose life was transformed by an act of migration, to be entirely objective about the value of such acts. I have spent much of my writing life celebrating the potential for creativity and renewal of the cultural encounters and frictions that have become commonplace in our much-transplanted world.

Then again, as people keep pointing out, I have a second axe to grind, because the Satanic Verses controversy was a pivotal moment in the forging of a British Muslim identity and political agenda. I did not fail to note the ironies: a secular work of art energized powerful communalist, anti-secularist forces, ?Muslim? instead of ?Asian?. And, yes, as a result the argument about multiculturalism for me has become an internal debate, a quarrel in the self.

Nor am I alone. The melange of culture is in us all, with its irreconcilable contradictions. In our swollen, polyglot cities ? ?the locus classicus of incompatible realities? one of the characters in The Satanic Verses calls them ? we are all cultural mestizos, and the argument within rages to some degree in us all.

So it is important to make a distinction between multifaceted culture and multiculturalism. In the age of mass migration and the internet, cultural plurality is an irreversible fact, like globalization. Like it or dislike it, it?s where we live, and the dream of a pure monoculture is at best an unattainable, nostalgic fantasy, and at worst a life-threatening menace ? when ideas of racial purity, religious purity or cultural purity turn into programmes of ?ethnic cleansing?, for example, or when Hindu fanatics in India attack the ?inauthenticity? of Indian Muslim experience, or when Islamic ideologues drive young people to die in the service of ?pure? faith, unadulterated by compassion or doubt.

?Purity? is a slogan that leads to segregations and explosions. Let us have no more of it. A little more impurity, please, a little less cleanliness, a little more dirt. We?ll all sleep easier in our beds.

Multiculturalism, however, has all too often become mere cultural relativism, a much less defensible proposition, under cover of which much that is reactionary and oppressive ? of women, for example ? can be justified.

The British multiculturalist idea of different cultures peacefully coexisting under the umbrella of a vaguely defined pax Britannica was seriously undermined by the July 7 bombers and the disaffected ghetto culture from which they sprang. Of the other available social models, the one-size-fits-all homogenizing of ?full assimilation? seems not only undesirable but unachievable, and what remains is the ?core values? approach to which Parekh alludes, and of which the ?Britishness test? is, at least as currently proposed, a grotesque comic parody.

When we, as individuals, pick and mix cultural elements for ourselves, we do not do so indiscriminately, but according to our natures. Societies, too, must retain the ability to discriminate, to reject as well as to accept, to value some things above others, and to insist on the acceptance of those values by all their members. This is the question of our time: how does a fractured community of multiple cultures decide what values it must share in order to cohere, and how can it insist on those values even when they clash with some citizens? traditions and beliefs?

The beginnings of an answer may be found by asking the question the other way around: What does a society owe to its citizens? The French riots demonstrate a stark truth. If people do not feel included in the national idea, their alienation will eventually turn to rage. Chouhan and others are right to insist that issues of social justice, racism and deprivation need urgently to be addressed. If we are to build a plural society on the foundation of what unites us, we must face up to what divides.

But the questions of core freedoms and primary loyalties can?t be ducked. No society, no matter how tolerant, can expect to thrive if its citizens don?t prize what their citizenship means ? if, when asked what they stand for as Frenchmen, as Indians, as Americans, as Britons, they cannot give a clear reply.