Path of Blood, a documentary about the modern totalitarianism of radical Islam, which with luck will be on general release soon, makes my point for me. Jonathan Hacker, the director, just shows al-Qaeda as it is. He offers the audience no characters to follow, no easy morals to draw and, above all, no explanations. "This is what terrorists and the police who fight them are like," he seems to say. "What more do you need to know?"

He is able to work with confidence because his executive producers, Abdulrahman Alrashed and Adel Alabulkarim, are what admiring reporters call brilliant blaggers. When Saudi special forces raided the hideouts of al-Qaeda terrorists, they seized hundreds of hours of footage — home movies, in effect, although the content was far from homely. The Saudi authorities had their own crime-scene footage of their troops fighting back. Alabulkarim and Alrashed somehow persuaded them to hand over both sets of tapes. 

We see shots of happy paramilitaries laughing with their friends and playing games. It takes a while to realise that they are happy because they are about to kill themselves and anyone else they can reach. We see scenes of men torturing and murdering an American hostage in an al-Qaeda safe house, while across the room a father teaches his little boys how to be soldiers of God. And we hear religion everywhere, from "scholars" telling their followers to emulate Muhammad's massacres of the Jews, to men thanking Allah for giving them the opportunity to kill. 

Admittedly, the film is a corrective to those in the West who insist that religious violence has nothing to do with religion. It is a god-soaked as much as a blood-soaked documentary. But Hacker is not interested in why extremists turn to a gruesome strain of Islam. He just shows them murdering and the Saudis retaliating by killing or "re-educating" them. 

I have read thousands of articles and seen thousands of television news reports on radical Islam. The rationalist fallacy has hobbled most of their authors. They believe that irrational movements must be a response to sensible or at least explicable grievances, which men and women of good will can address. Their examination of the evidence may be level-headed. Their willingness to balance competing opinions may be praiseworthy. But they never stop to think that educated Westerners may be unable by temperament and training to accept that unhinged sectarianism will not be stopped by concessions.

Usually, the rational explanation they offer is that violence is a reaction to some crime of the West. But it does not matter what the "root cause" on offer is: the gap between comprehensible grievances and the violence inflicted by a psychotic global movement or indeed a psychotic global war is too great for reason to bridge. Like Niall Ferguson, you can say you should stay out of its way, which may be impossible. Like the Saudi monarchy, you can say you have no choice but to fight it, which may be a mistake too. But to pretend that catastrophic events or movements are no different from the world's ordinary troubles is to make the biggest mistake of all.