What turns a British citizen into a suicide bomber? It is one of the most difficult, and most important, questions for MI5. That's why MI5's recent report on how to answer it is so interesting: it gives an insight into how the security service is responding to its most significant challenge.
The report hasn't been published, but judging by the summary that leaked last week, MI5's thinking is, to put it mildly, disappointing: it mixes the obvious with the tendentious to produce the fatuous.
There is, the report insists, "no single pathway to extremism", which is very obvious. As with all serious crimes, if you look closely at the perpetrators, they each came to perform their evil deeds for their own unique reasons: each murderer, rapist, paedophile and terrorist is different from every other.
aWhile true, that banal observation is of no help whatever in combatting murder, rape or terrorism - so it is surprising to see it emphasised in a report from MI5's "behavioural science unit".
The report goes on to identify what it calls "key vulnerabilities" that lead a person to terrorism. These include "the experience of migrating to Britain, facing marginalisation and racism [and] the failure of those with degrees to achieve anything but low-grade jobs".
The report even throws in "inadequate media coverage" that "perpetuates negative stereotypes" as a catalyst for terrorism.
This is plain silly. Those experiences are not "key vulnerabilities", because thousands, indeed millions, of immigrants to Britain go through them, often in more extreme forms than anything that affected the 7/7 suicide bombers, without turning into terrorists.
MI5 might as well say that poverty is a "key vulnerability" that turns a man into a rapist or a murderer, although it is perfectly obvious that most poor people aren't killers or rapists.
But that is not the most bizarre feature of the MI5 report.
It conspicuously fails to mention the potential terrorist's most obvious "vulnerability": adherence to an extremist form of radical Islamism.
It stresses that "far from being religious zealots, a large number of those involved in terrorism do not practice their faith regularly; many lack religious literacy and could be regarded as religious novices." Which may be true, but does not alter the fundamental fact that all of the suicide bombers in the UK, and all of those who have been involved in helping them plan and execute their attacks, have claimed that they were motivated by commitment to an extreme form of Islam.
No one from any other faith has perpetrated a suicide bombing in Britain, or said anything to suggest that such terrorism would or could be justified. Does MI5's behavioural science unit really believe that, if the ideology of extreme Islamism did not exist, the people who blew themselves up on July 7, 2005 would have found another justification for it and done it anyway?
MI5's claim is all the more strange because other European security services have published detailed reports that take the connection between an extremist form of Islam and terrorist violence for granted.
The Dutch equivalent of MI5, for instance, has published several investigations into what makes a terrorist. Those reports agree with MI5 that there isn't a simple check-list that tells you who will become a terrorist. But the Dutch don't try to dodge the obvious correlation between adherence to extremist radical Islam and terrorist violence. Instead, they examine the forms that extremism takes and look at ways of stopping its spread.
On the evidence of its report, MI5 is reluctant to do the same. Why? I know from talking to people who work for the organisation that it is deeply split on the matter.
One faction of the security service - represented by the recent report - insists that there is no intrinsic connection between radical, extremist Islam and terrorism in Britain.
Another insists that there is: without Islamism, while there might of course be terrorism in Britain at the moment, it would not take the distinctive form of suicide bombing, which we have all come to dread.
People in MI5 tell me that denying the connection between Islamism and terrorism derives from the belief that if you accept it, there's no hope for a multicultural society in Britain: we would just have to recognise that part of the population is permanently liable to become terrorists.
But that conclusion is a mistake - as big a mistake as it was to conclude, from the plots hatched by some fanatical Catholics against Protestant rulers in the 17th century, that there was something intrinsic to Catholicism that made it impossible for Catholics to live in peace with Protestants.
I hope the debate within MI5 gets resolved soon. Because if MI5 starts devoting resources to following Hindus and Sikhs in the expectation of identifying the next suicide bomber in Britain, it will certainly be wasting its time and our money, as well as placing us all in much greater danger.