Britain prides itself on peaceful multiculturalism. By international standards, our race relations have long been harmonious, in spite of high levels of immigration and - at least in the cities - significant ethnic and linguistic diversity. Any success of the British National Party is seen by mainstream politicians and the media as a blow to the liberal body politic. Racism is simply not cricket.
The London terrorist bombing campaigns will put this tolerance to the test. When British Moslems blow themselves up - along with dozens of innocent fellow citizens - the only Brits rubbing their hands are the BNP. But if tolerance and understanding triumph over fear and loathing, it is certain that the case for a diverse, multi-ethnic society will have to be restated, and with more conviction.
This means having an honest debate about the benefits and costs of living in an ethnically diverse society. The cause of multiculturalism is not served by woolly, liberal assumptions that everything about diversity is wonderful. The case for immigration and diversity is strong enough. It is only weakened by flaky embellishments that diverse societies are more creative or happier than homogenous ones.
Richard Florida, author of The Flight of the Creative Class, has made a compelling case that creativity is vital for economic success and that creative people are attracted to liberal, diverse societies. It is a wonderful thought that if only the world was more like San Francisco, creativity would be unleashed globally, but evidence for the second half of this proposition is weak.
Florida's thesis is popular among Labour ministers, because it takes two social democratic goodies - diversity and creativity - and suggests that they go hand-in-hand. But as thoughtful critics of Florida like Charles Leadbeater point out, many countries with a strong track record in innovation are homogenous, insular and conservative: Finland, Sweden, Israel and Ireland. Indeed, if innovation increasingly rests on collaboration, societies with strong shared codes and cultures may do better than diverse ones.
An activist state seems to be much more important than a cultural melting pot. Chinese investment in research and development ranks at number three in the world, after Japan and the US, and is rising by 23% a year. China is not an open, tolerant democracy of the kind eulogised by Florida, yet seems to be doing well on the creativity front. Singapore and Korea lead the way in biotech, unconstrained by liberal ideas about animal rights or squeamishness about cloning, genetic modification and stem-cell research.
History is not on Florida's side, either. The creative breakthroughs of the 19th century in science, philosophy, music and visual art came from the group stigmatised in modern literature departments as DWEMs: Dead White European Males, with bourgeois backgrounds.
Nor is there any evidence that multi-ethnic societies are happier. I much prefer to live in a diverse culture and for my children to grow up knowing that people come in all colours. But it's a mistake to extrapolate from individual preference to social good. The evidence on happiness from regional and national studies suggests that, if anything, socially and ethnically homogenous communities are happier than mixed ones.
Of course, it may be that people's racism is lowering their happiness levels - in which case, tough. Or that even peaceful communities live with a degree of wariness or fear of each other. There may be ancient anxieties about people who are visibly different from us.
Britain is now a hugely diverse society, and although the evidence on creativity and happiness is far from conclusive, a diverse society is better than a uniform one on clear economic grounds. But it insults everyone's intelligence to pretend that those benefits don't bring any costs. It doesn't help that the Government has been so reticent about its achievements in immigration. Since 1997, a million non-British immigrants have been added to the citizenry of the country, accounting for the bulk of population growth. During these years, the UK economy has enjoyed its longest sustained period of expansion for centuries.
The experience of the past decade nails all the pseudo-economic arguments against immigration. They'll take our jobs! (unemployment has been at historically low levels). They'll scrounge off the state! (immigrants contribute £2.5 billion more to government coffers than they receive in benefits). They'll slow down the economy! (GDP annual growth has been a quarter-point higher as a result of immigration, according to the Treasury).
They'll be a drain on the health service! (the NHS could no longer operate without immigrant labour, at all levels). Immigrants are also young, which addresses the future imbalance between pensioners and taxpayers.
Our economy, tax revenues and public services are all much better as a result of a relatively liberal immigration policy. This alone is enough to support an open society, without straining to prove that everything in the garden is rosy.
But in the end, there is a limit to the use of the self-interest argument.
Even if immigration was not known to be good for the economy, it is certainly better for the immigrants. If we want to live in a global economy, we have to start looking at moral responsibility in global terms, too. Whether or not immigration is good, it is right.
Richard Reeves is director of Intelligence Agency, an ideas consultancy; e-mail: firstname.lastname@example.org.