Britain's spies should be more open, the think-tank Demos proposes in a new report today. There are three objections to this, not all of which are entirely consistent with each other. The first is that part of the point of the espionage business is secrecy. A central purpose of intelligence is to know about threats to our national security without the authors of those threats knowing that we know. The second is that the story of recent decades, and certainly since the end of the cold war in 1989, is that of increasing transparency. We have come a long way from the days when the secret services officially did not exist, or spend public money, and journalism was censored by D-notices.

Now we know who heads the security services: Jonathan Evans is the head of MI5, the domestic service, and Sir John Sawers is the head of MI6, its foreign counterpart. Their work is overseen by the Intelligence and Security Committee of Privy Councillors, appointed by the Prime Minister but reporting to Parliament. And much of the counter-terrorist work against al-Qa'ida-sympathising extremism is necessarily out in the open, as a public persuasion campaign. Of course, the one controversial aspect of this openness was Tony Blair's decision eight years ago to publish a dossier summarising the intelligence on Iraq's weapons of mass destruction in order to make his case for military action. But the mistake there was in the contents of the assessments and the way they were rewritten for a political purpose, rather than the principle of publication.

This trend towards greater openness is sensible and right. In this respect, we agree with the Demos report. The security services should be as open and as accountable as is possible, consistent with such secrecy as is necessary. There is a little way further to go in this, as is illustrated by the travails of David Miliband, the Labour front-runner who is interviewed in The Independent on Sunday today. To be fair to him, when he became Foreign Secretary in 2007, he received a difficult inheritance. The primary responsibility for the security services at the time lay with his predecessor Jack Straw and with David Blunkett (as Home Secretary, he was responsible for MI5). We are sceptical about whether they tried hard enough to find out whether their public servants had obeyed the absolute prohibition on torture. That prohibition includes a ban on any use of intelligence obtained by torture, and there is an obligation to report such torture when perpetrated by others. Mr Miliband's attempts to get to the bottom of what the Americans did and how much our spies knew about it were complicated by the need to maintain relations with our ally.

So there is more to be done in tightening up oversight of the agencies to ensure that such lapses are less likely to occur in future, and David Cameron should act promptly on the review by Sir Peter Gibson into allegations of complicity in torture.

However, openness is not the antidote to every ill, and our third objection to the Demos report is that it is naive to believe that greater transparency would inhibit internet conspiracy theories. Inevitably, many of the popular theories ascribe important roles to the secret services of the US, Britain and Israel, and impute malign motives to them. The Independent on Sunday has recently published articles by Tom Mangold, the investigative journalist, listing the many utterly unlikely things that would have had to have happened if David Kelly had been murdered. We think it is important that the so-called mainstream media should continue to hold up such fantasies to scrutiny and, indeed, ridicule.

But it is not mainly up to governments to confound conspiracy theories such as the 9/11 "truth" movement and the Protocols of the Elders of Zion, a hateful anti-Semitic fairytale peddled by many al-Qa'ida sympathisers. Demos suggests that the Government should "openly infiltrate" websites, "flagging dubious evidence and disputing theories [to] encourage people to question and doubt false information online", but it accepts that "there is a limit to what the Government can do". That is an understatement, given the typical psychology of conspiracy theorists, which includes a deep mistrust of the institutions of authority.

Ultimately, education and information are the only solutions, and the internet is simply a means of communication rather than a step back to a dark age. There were grassy knolls and faked moon landings before the worldwide web. But openness is the right principle. Demos deserves one cheer out of three.