One thing is sure: 42 days solves nothing

The argument against six-week detention without charge is partly, of course, that it has a significant effect on liberty. But the other part of the argument against it is that it will have no significant effect whatever on terrorism. That in making first 56, and now 42, days their main anti-terrorism policy objective for the whole of the past year, ministers have simply got their priorities wrong.

No one disputes that we face a serious terrorist threat - though there is plenty of dispute about quite how serious. But if there are, as Mr Brown claims, 2,000 active terrorists in the UK, why is the Government straining every sinew to obtain a power which ministers insist will be applied only to a handful of people, in the most exceptional circumstances, if at all?

Even the Home Secretary, Jacqui Smith, does not claim that being able to detain someone an extra two weeks will make all the difference. Nor can she produce a single example of any anti-terror investigation suffering for lack of the power. The most she will say is that it might come in handy in the future. But instead of policies that might be useful in some unspecified future circumstance, what about some policies that might be useful now?

A better use of parliamentary time and officials' energies would be devising policies to reach the alleged 2,000 terrorists, and the hundreds of thousands of potential further recruits to their number: policies of outreach, deradicalisation, intelligence. We are doing those things, too, the Government will protest. But the totally mistaken focus on detention and policing risks sabotaging them, creating the very alienation on which terrorism feeds.

Terrorism's main purpose is not to kill - it is to terrorise. Terrorism works not because of what the terrorists can do to us - which is relatively limited - but because of what they can panic us into doing to ourselves. "42 days" is a self-inflicted attack on our free society which is precisely furthering the aims of the terrorists.

We cannot refuse to be killed. With or without 42 days, there will be further attacks on London. But we can refuse to be terrorised. We should be building defences in our minds against terror. Rather than fuelling disproportionate, uninformed fear in pursuit of their police-powers agenda, the Government-should be educating people about the true nature of the threat. They should tell us that it is grave, but not devastating. They should acknowledge, for instance, that most so-called "weapons of mass destruction" are nothing of the sort.

It is certainly easy enough for amateurs to build a chemical or biological weapon which could kill a few people, like the nutters in Japan did 13 years ago. The chemicals are not impossible to get. Some bio-agents are, by definition, naturally available. But it takes sophisticated technology and factories to make a chemical or biological weapon capable of killing on a mass scale. Such factories could not go undetected.

The three most recent high-profile plots - the Exeter bomb, the Glasgow Airport and Haymarket attacks, and the alleged airline bombing plot of 2006 - have all been notable, in varying degrees, for their amateurism. Did the authorities point this out? No, they tended to play up their seriousness. In the attempted Exeter attack last month, they gave an extraordinary amount of information about the main suspect's alleged links with "Islamic extremists" before he was even charged. Could that have been entirely unconnected with the 42-day battle?

Last year's Haymarket incident was described as involving a "potentially viable device" - in other words, not a bomb at all, only a potential one. And information published in the United States suggests that the most serious of the three cases, the 2006 alleged airline plot, for which several men are currently on trial, could possibly not have been quite so blood-curdling as was claimed.

The fact is that though the terrorists want to kill us, they cannot do it very often. Even less often can they do it very effectively. The Government is asking for a detention period six times longer than it needed in 2003 but has presented no evidence whatever that the terrorist threat is six times greater than it was then. The threat must be kept in proportion, as must the response to it.

Why do governments so often tilt at windmills? Not just over detention without charge, but over, for instance, ID cards - another pointless and damaging policy. Partly it is posturing: they think it will make them look tough. But it never does. Tony Blair got no credit for trying to push through his even more draconian 90-day detention plan. ID cards have caused ministers nothing but grief.

And even if Mr Brown wins the vote next week, his split-the-difference reduction in the detention period from the arbitrary 90 days to an arbitrary 56, then an arbitrary 42, and now his late flurry of meaningless concessions, have undermined any attempt to present it as a victory for toughness, or principle, or for anything other than political haggling.

Partly it is that governments long for easy and simple answers to our problems. But policies which do not work are never simple or easy. And partly it is pride - they dare not retreat, even when they know they are wrong, because they would lose too much face. So they waste even more time desperately trying to shore up failure. What we can categorically say that this has absolutely nothing to do with is security.