A collision with the errors of messianic religion is, however, the last thing liberal Europeans want. At the underwear bomber's university, academics and student leaders alike were not prepared to argue with radical Islamists to give onlookers a livelier impression of the truth. They wanted to pretend that radical Islam did not exist and slink away from an urgent confrontation. As I watched Sands affect indifference and Streeting sneer and jeer, the thought that sprang to my mind was they were cowards, with a fear so deep they dare not admit it.

I can see no more important task at present than working out how European liberalism has gone so badly wrong. Why does a culture that prides itself on its opposition to bigotry become so feeble when it confronts bigots dressed in the black robes of clerical reaction? Until we understand, we cannot cure, and there is an emerging understanding among those who worry about the dark turn liberals have taken that Western guilt lies at the root of their moral failure.

After the Abdulmutallab case led the US to impose travel restriction on Nigeria, the Nigerian Nobel laureate Wole Soyinka said Britain should be the Americans' target. It was "a cesspit", he cried, the "breeding ground of fundamentalist Muslims". Nigerian national pride distorted his thinking. He could not seem to acknowledge that the religious war between Christians and Muslims in his own country was proceeding without British help. But one point he raised is being echoed by many others. Soyinka identified an inverse snobbery behind Britain's failure to argue against its enemies. Colonialism bred an innate arrogance, he said. But after the imperial adventure ended, arrogance produced a pride in openness so intense that it confirmed Britain's belief that it was still a morally great country.

Last month, Pascal Bruckner's magnificent The Tyranny of Guilt: An Essay on Western Masochism (Princeton) was published. Like the Nigerian novelist, the French philosopher sees an unacknowledged arrogance — "a blatant case of imperialism in reverse" — hiding behind the cries of Western liberals that we are the root cause of every psychopathic movement and regime on the planet.

"The whole paradox of sobered-up Europe is that it is no less arrogant than imperial Europe because it continues to project its categories on the rest of the world and childishly boasts that it is the origin of all the ills that beset mankind. Our superiority complex has taken refuge in the perpetual avowal of our sins, a strange way of inflating our puny selves to global dimensions."

Bruckner might have added that the consequences for Muslims are dire. Instead of meeting a confident liberalism in London, Abdulmutallab found a culture that insisted that the West offered a just cause for a war against itself; that declared he had every right to be furious, to despise liberal values as fraudulent and to believe in the moral superiority of radical reaction. Ayaan Hirsi Ali, whose contemptuous treatment by liberal European thinkers was one of the defining cultural events of the last decade, is equally clear that middle-class guilt explains why the dons of Oxford University wanted to shun her and employ Tariq Ramadan. When she arrived in Holland, fleeing the failed Muslim culture of Somalia, the extent of Europeans' scorn for the admirable civilisation they had built astonished her.