Their answer is 42. What, exactly, is the question?

Most reports say that Jacqui Smith enhanced her political reputation at Monday night's weekly meeting of Labour MPs. The suggestion cast an odd sidelight on the nature, these days, of the Home Secretary's party and government. It seems you can advance your career best just by adding another brick to the emerging edifice of the security state.

Ms Smith wouldn't see it that way. On Monday, she was merely persuading some of perhaps 50 sceptical MPs that the government would resort to six-week detention without trial only in moments of dire emergency. She was making concessions to troubled consciences. She was reassuring colleagues that a draconian measure would be applied only with judicial oversight and parliamentary scrutiny.

Some MPs are said to have been persuaded ahead of next week's vote. Granted, if you believe that your job is to listen hard to whatever the government decides to tell you, if you want a judge involved, and if you do not wish to see your Prime Minister damaged, Ms Smith's promises might do the trick. The establishment of a permanent state of emergency in Britain could even sound like a price worth paying.

The police do not see a need for 42 days, and nor does the director of public prosecutions. Lord Goldsmith, the former Attorney-General, pliant enough in his advice on the legality of the Iraq war, is opposed, as are the Tories and the Liberals. Lawyers generally, to put it mildly, see no justification. But the government is determined.

Determined for what reason? You could argue, as Gordon Brown did in a newspaper article on Monday, that the terrorist threat is great, far-reaching and, above all, without precedent. You could also choose to believe that the Prime Minister has pinned his tattered colours to this mast and cannot afford to risk retreat or defeat. Neither argument has much to do with practicalities or principle.

Why 42 days, no more and no less? The government will not budge on it, yet the Home Office cannot say why this number is essential. Why, equally, does Britain require a period of detention without trial far in excess of anything sought by any other democracy? Even the existing 28-day power goes far beyond anything in use in Europe or - a mere two days - in America. Is the threat to Britain actually unique? We are not told.

Mr Brown, in his newspaper article, did his best to make the threat sound vast. Trouble was, he deployed the now-standard round numbers MI5 produces regularly: 2000 suspects; 200 networks or cells; 30 active plots. After the London bombings, and after Glasgow Airport, I am not daft enough to doubt that threats exist. But such estimates make me suspicious.

Take those "30 active plots". What might those involve? Given the context, nothing less than conspiracy to murder. So the security services have knowledge of 30 separate conspiracies to kill and yet fail to act? Why so? Presumably because they possess "knowledge" but no evidence that would stand up in court. This suggests that the aim is to detain for up to 42 days on suspicion. In Britain, this counts as a departure.

Mr Brown did not address the point. Instead, he wrote of the fiendish complexity of modern, computer-literate terrorism, with its hundreds of hard drives and discs requiring analysis, its encryption codes, and its trans-continental internet connections. It takes a lot of work - but apparently no more than 42 days - to wade through that lot. Yet no other country makes any such argument. That counts as strange.

The Prime Minister still offers what he calls "practical safeguards against arbitrary treatment". The current betting is that enough of his MPs will accept the latest version of those and give him his 42 days. In some government chatter, indeed, there is the suggestion that the need for detention above 28 days would arise in circumstances so remote they are almost theoretical. It might never happen, MPs have been told. So do we suspend civil liberties because the threat is actual and terrible, or merely theoretical?

Oddly enough, much of the power the government seeks already exists. It is set out in a thing called the Civil Contingencies Act 2004. This even provides for 58-day detention. The government argues, however, that this legislation - its own - was designed to enhance civil defence and is therefore somehow inappropriate for terrorism. What they really mean is that under the act they would have to declare a state of emergency publicly. The 42-day plan would instead blend, they hope, into the woodwork of general awareness.

It would be "reviewed" annually, of course. The same was said, once upon a time, about the "temporary" Prevention of Terrorism (PTA) Act that Labour in opposition promised to abolish. The 42-day plan would not be used for "routine" investigations. That was also a promise made about the PTA. Still, MPs would have a vote on the introduction of the power a week after the fact, and another vote within 30 days. This is a "concession".

MPs like to be consulted; they should be consulted. But on what basis of knowledge would they judge during a real or alleged emergency? Ms Smith suggests that she could provide confidential information to some, but not all, of them. The rest would have to be satisfied if the Attorney-General and the judiciary were satisfied. The Home Secretary has no desire, meanwhile, to have the new power in place for more than "a limited period of time". Back to the PTA, I think.

You could recall, of course, that the IRA demonstrated its intent and ability often enough, and certainly more often than any Islamist zealot has managed. Yet according to Mr Brown the threat this time is far greater than anything ever posed by Irish Republicans. On current evidence, mercifully, that simply isn't true, but the 2008 anti-terror bill contains provisions - secret evidence withheld from defendants, for example - never before contemplated.

This government has produced rafts of legislation said to be designed to protect us against terrorist authorities. Each absolutely essential act is soon supplanted by a fresh absolutely essential act. The state's only recent failure came with Tony Blair's outrageous attempt to establish 90-day imprisonment without charge. New Labour were supposed to think again and, in a way, they did: so 42 days it is.

What gets overlooked, of course, are all the other little bits and pieces of new security law smuggled in during rows over the headline issues. Offences "associated" with terrorism, no matter how trivial, no matter the meaning of "associated": those should provide work for the European Court of Human Rights. What does it prove if a Muslim youth, who wants to understand the arguments, downloads a web page? That will depend on the state. What does it prove if I "communicate information" about the armed forces here or in Afghanistan? What information, exactly?

They chip away and they chip away. They intend to keep us safe. They do not ask, for they do not wish to ask, whether any free society can ever be entirely safe. And, worst of all, they go on creating enemies for the liberties they say they aim to protect. While regretfully, with heavy hearts, removing those liberties.