There are many reasons why governments hold public inquiries: to establish what really happened when the circumstances are murky; to stem public disquiet; to foster confidence in the institutions of power, and - most important of all, perhaps - to chart lessons for the future. The London bombings of 7 July would seem to meet all these criteria amply.
It just so happens that these bombings also constituted the worst ever terrorist attack in Britain. Here was a horrific and highly disruptive episode in the life of this country. Fifty-two people were killed; dozens were injured. Public confidence in the security of the capital and the safety of public transport was undermined. Community relations were impaired - though, thankfully, not as catastrophically as might have been feared. And while much about the bombings and their aftermath is now known, a great deal is not.
Yet after suggesting that it was considering a public inquiry, the Government has now decided against. The Home Office broke the news earlier this week, softly, softly, saying that a "narrative" of events would be compiled by a senior civil servant and presented to Parliament instead. Tony Blair defended the decision to unhappy opposition MPs at Prime Minister's Questions, saying that it was already known "essentially" what happened on 7 July and that an inquiry would divert "a massive amount of police and security service time".
Then, yesterday, the Commissioner of the Metropolitan Police, Sir Ian Blair, added his two ha'porth. The police, he said, were "flat out" continuing the criminal inquiry into the bombings and trying to prevent further attacks. He described the "narrative" idea as a good compromise, but added that it would have to try to answer some of the questions people had raised.
Which implicitly makes the point that all those calling for a public inquiry have argued all along. The Prime Minister may be satisfied that what happened on 7 July is "essentially" known, but very many other people are not. Sir Ian mentioned two specific questions that have not, as yet, been answered: did the authorities know anything about plans for the attacks in advance, and was there - given the explosives found in a car at Luton station - a fifth bomber?
There are, of course, plenty of other questions. Why was it stated so quickly and confidently that none of the four suicide bombers was known to the security services, when it later emerged that the presumed ringleader had been the subject of an investigation? Was the lack of intelligence as abysmal as it appears to have been and, if it was, was this because of a shortage of funds or poor direction or both - and what has been done to address this? And why was Britain's state of alert downgraded a month before the attacks?
The response of the emergency services - and of the public caught up in the attacks - was, by all accounts, admirable. But reports spoke of a problem with some communications equipment and there was confusion about which agency had the authority to block mobile phone signals. Individual and collective resourcefulness is something to take pride in, but there must also be lessons to be learnt. It is simply no good for the Met to say that its officers are too hard-pressed to take part.
A public inquiry would be the most appropriate forum for all aspects of 7 July to be considered. But it is essential for another reason, too. Anything that is kept within the government machine will inevitably be tarnished by memories of the assurances we were given about Iraq's weapons and all that emerged about Mr Blair's style of government from the Hutton and Butler inquiries. Trust is this government's Achilles heel. Only a full public inquiry will carry conviction.