These are difficult times for multiculturalism, especially in Europe. Reasonable people are having misgivings about the cultural and religious practices of immigrant communities in their midst. People say things like, "I used to be for openness and toleration of difference, but now I see where it's leading." Where is it leading? This is all about Islam. Rather simple requests, like those of schoolgirls to wear headscarves, are suddenly freighted with immense significance. The feeling is often that this sort of seemingly simple proposal is really part of a larger, more sinister package.

The package is Islam, and it includes such terrible things as we see in the press daily: things happening, for instance, in Nigeria or Saudi Arabia. If you argue that the girls in question aren't living in Nigeria or Saudi Arabia, and almost certainly don't share, say, extreme Wahabi views, people will look at you with that kind of almost-indulgent pity reserved for the terminally naive, or they will tell you stories about how imams are twisting the girls" arms, making them into unwilling stalking horses for Islam.

You can't just talk about headscarves as an issue on its own, and all the sociological evidence about the (in fact quite varied) motives of the girls themselves is swept aside as irrelevant. This is a classic example of block thinking, which seems to have made huge strides in Europe in recent years. John Bowen's forthcoming book, Why the French Don't Like Headscarves, documents this shift in the French case.

Multiculturalism, in order to be effective within a liberal democratic polity, has always carried a double requirement: first, that cultural differences must be recognized and tolerated; and second, that there must also be an awareness that minority cultures are no more monolithic and no more free of internal differences and contestations than are our own. Failure to grasp this on an existential level (which is emblematic of block thinking) leaves avowals of recognition and tolerance quite hollow. They would crumble swiftly at the slightest hint of any confrontation that asks for a reconsideration of our public norms and practices. This simply means that multiculturalism and block thinking cannot coexist in the long run.

Block thinking fuses a varied reality into a single indissoluble unity, and it does this on two dimensions: first, the different manifestations of Islamic piety or culture are seen as alternative ways of expressing the same core meanings; and second, all the members of this religion or culture are seen as endorsing these core meanings. That a girl's decision to wear the headscarf might actually express a rebellion against her parents and their kind of Islam, or that some of these girls might be deeply pious and yet utterly revolted by gender discrimination or violence, is all lost from view.

Block thinking is an age-old phenomenon, and we all do it to some degree. In another age we might be indulgent, but today such a posture has explosive potential. Block thinkers are prime recruits for Samuel Huntington's theory of the "clash of civilizations" and its attendant scenarios. What's worse, the actions that result from block thinking tend to edge us closer to this very sort of nightmare scenario. By treating all of the varied segments of Islam as though they constituted the same single threat, such thinkers make it harder for Muslims to stand out and criticize their own block thinkers, who are themselves busy fashioning a gigantic, unified enemy. "Christians and Jews," says Osama bin Laden—that takes in quite a lot of people. Block thinkers on each side give aid and comfort to block thinkers on the other, and each exchange edges us closer to an abyss. We're still very far from the edge, but the sooner we stop this madness, the better.