It seems to be a law of politics that the less the evidence for a particular proposed law or measure, the more vehement and blood-chilling are the arguments adduced for its imposition. That is certainly the case with the Home Office's increasingly desperate attempts to introduce 42 days detention without charge.
Today it was the turn of the utterly compromised and partisan Sir Ian Blair to justify the proposal before the Commons committee examining the counter-terrorism bill. He admitted to the MPs that there was no evidence to justify the government's claim that the police need the extra time. Indeed, he said, it had never been part of the police case that there was any such evidence. Rather (and this is also the Home Office line, of course) the situation might possibly arise in the future. And it's better to take the powers now, just in case.
Blair added, as he invariably does, that with the "increasing complexity and sophistication" of terrorist plots the day would soon come when they did need 42 days detention pre-charge.
There have been a number of cases where the level of threat that we perceive means we make an arrest when we have almost no evidential material at all. So we are starting from a place where we are very very concerned about what these people are going to do but we are not quite sure what it is.
Which for some reason made me start thinking about Jean Charles de Menezes.
As an exercise of the "precautionary principle", this really takes the biscuit. I'm reminded of the White Knight in Through the Looking Glass, who had equipped his horse with a mouse-trap. When Alice protested that there weren't likely to be many mice on his horse's back, the knight agreed. "But if they do come," he added, "I don't choose to have them running all about."
He had also attached devices to the horse's legs "to guard against attacks from sharks".
The case for 42 days is based almost entirely on supposition and scare-mongering. In the language of ministers and their police allies, terrorist plots are always multiplying and deepening, the threat is always unprecedented, the sophistication of the terrorists is invariably growing. Faced with this menace, the authorities of law and order present themselves as weak, powerless and vulnerable, fighting a rearguard action against mighty odds. Society itself, in this analysis, is fragile and perennially imperilled. Only by increasing detention, increasing surveillance and eroding rights can life even be sustained here.
But what about the IRA? Ah, says Commissioner Blair, they didn't have mobile phones or the Internet back then, did they? And they usually phoned in warnings. And they weren't into suicide bombing. Altogether a better class of terrorist.
Such nice, friendly, unthreatening terrorists were the IRA that they managed to kill an estimated 1800 people during a thirty year campaign. Which is an average of 60 per year. This contrasts with the 52 victims of the London tube bombings. If we say that their "campaign" has been going since 2001, then in all but one of those eight years there have been no deaths at all in Britain. The IRA were so gentlemanly that they came within an ace of murdering the entire cabinet in 1984 (and, as I recall, there was no warning). They were a real threat, and we got through it.
It's been pointed out often enough by opponents of the extension that other countries - including the US, Canada, Australia and most of continental Europe - seem to manage perfectly well without such a draconian regime. Sir Ian and his cronies have finally, it seems, found an answer to this one. Britain, he claimed today, was uniquely vulnerable. "We appear to face the most radical and escalating threat of any of these countries in terms of the number of people involved and plots going on and that are happening now," he claimed.
If this is true, perhaps the police and the government should be concentrating on finding out why, rather than looking to yet more laws.
Blair in any case seemed curiously vague about the details and seriousness of these plots. He said that a total of 12 terrorist plots, all "likely" to involve loss of life, had been foiled since the successful bombings of 2005. His colleague Bob Quick, sitting next to him, put the figure at 15.
In the same period there have been around 2200 murders in England and Wales, and as many as 40000 recorded rapes. And it's worth noting that the police have somehow managed to detect all of these 15 terrorist plots, even while labouring under the massive disadvantage of not being able to detain suspects for more than a month before charging them with one of the many new and remarkably flexible terrorist offences.
Blair's case seemed to be based, at least partly, on making the police's life easier. "There are practicalities - where officers have to trail all around the world, with very extreme and tight deadlines, working with high technology teams breaking encryption codes," he said. "That places real pressure on people to deliver the evidence."
I'm sorry, but enabling police officers to knock off work early on a Friday afternoon is not a convincing reason for further eroding our civil liberties.
In addition, Blair said, while you basically knew where you were with the IRA, these new terrorists can pop up anywhere, "out of left field".
Presumably he's thinking about cases such as the increasingly bizarre story of Andrew Ibrahim, the Bristol public schoolboy turned crackhead turned Big Issue seller turned devout Muslim turned (perhaps) would-be jihadist. Since Ibrahim disappeared into the legal limbo of pre-charge detention last week, the police have carried out three controlled explosions at his bedsit. But while information about the teenager himself has been plentiful very little has emerged about the nature or extent of his involvement in terrorism. It would seem that the police were acting on a tip-off from a suspicious local Muslim when they arrested him.
Someone who had lived at the same hostel as Ibrahim in his days as a spiky-haired drug addict told the Bristol Evening News that he had "clearly gone through a dramatic transformation":
We knew he was a Muslim but he was obviously now taking his faith very seriously. He was previously fanatical about hip-hop music. Now it seems to be religion that became his obsession.
Strangely, this seems to many observers to amount to proof of his guilt. I have no idea. But the notion that casualties of life like Andrew Ibrahim represent an existential threat of the order of Communism or the Third Reich - or even the IRA - strikes me as more than a little far-fetched.