Among the articles commemorating the fifth anniversary of the terrorist attacks of September 11 2001, the most disheartening are to be found on the website of an American organisation called Scholars for 9/11 Truth. The group claims that: “The [US] government not only permitted 9/11 to occur but may even have orchestrated these events to facilitate its political agenda.” That such opinions are disheartening does not make them surprising. Those who say “the World Trade Center was almost certainly brought down by controlled demolitions” – like those who say there were no gas chambers at Auschwitz – are often accused of inhumanity. But their motives, conscious or unconscious, are very human indeed. New facts impose new obligations. You are a bit less free the moment after you discover what Auschwitz was than you were the moment before. Some small percentage of people, unwilling to shoulder the burdens that knowledge imposes, will simply recoil from the knowledge.

Denial that the attacks of 9/11 happened remains a fringe phenomenon. But insistence that they mean little of consequence is becoming, in parts of the west, the consensus. “The west has been largely unchanged by the menace of Islamist terror,” Henry Porter, the columnist, wrote approvingly in The Guardian last week. For Mr Porter: “The radicalisation of Muslim populations in the west is so far Osama [Bin Laden]’s greatest achievement.” But this is a big deal. Such a radicalisation was scarcely thinkable five years ago. Why should the eruption of a violent and anti-western movement, with significant support in the world’s most active religion, not change the west?