Anyone who wanted to hear about GCHQ and its internet spying programme, Tempora, or whether MI6 is still in the illegal rendition business, or about the number of files held on the rest of us by MI5, could probably have spent Thursday afternoon putting up a few shelves.

Disclosures were not on the agenda.

Instead, the heads of the services spent a fruitful couple of hours, courtesy of Westminster's Intelligence and Security Committee, explaining that we shouldn't want to know such things, or setting out why they couldn't tell us about all the things they couldn't tell us, especially the things we don't need to know. It was an advert for the spooks' high opinion of themselves and their work.

Three middle-aged white men emerged from what one called the "ring of secrecy" to argue for the preservation of the ring against scrutiny, whistleblowers and journalism. GCHQ's Sir Iain Lobban, MI5's Andrew Parker and Sir John Sawers of MI6 were less grilled than lightly warmed over by Sir Malcolm Rifkind's committee. Proper evidence, the TV audience was told repeatedly, would only be revealed in private sessions.

The public hearing was a publicity stunt, in short, one that must have seemed like an excellent idea when it was mooted. That was before the American analyst Edward Snowden emerged from his country's National Security Agency with evidence of his own, evidence demonstrating that Tempora, for one example, is Britain's contribution to the effort to blow a gigantic hole in the internet and in privacy.

The Intelligence and Security Committee made a few dutiful noises about that, but in truth it is not the sort of body to fail to take the spymasters at their word. All involved are establishment trustees. Contrary to any impression given, theirs is not a parliamentary committee but - a crucial distinction - a committee of parliamentarians, peers and MPs, one largely chosen by the Government. Rifkind, as foreign secretary, once had charge of both GCHQ and MI6. For him, scepticism comes hard.

The three wise men were therefore given a free hand to claim public service as their highest duty, to deny illegality, and to ram home one underlying thesis. Secrecy is its own justification; it is so because we say so; trust us. Anyone who doubts - whether because of Tempora or the faked intelligence that procured the Iraq war - is damaging the effort to keep Britain safe. Journalism is a special menace.

You could believe it all if you wished. Many do. Given the self-evident fact of terrorism, that might be the safest course. But faith in the goodwill of those to whom enormous power (and £2 billion a year) is granted hardly resolves the paradoxes.

For example, Lobban said: "We do not spend our time listening to the telephone calls or reading the emails of the majority. That would not be proportionate, that would not be legal, and we would not do it."

You could call that reassuring. But you could also observe, first, that we only have Lobban's word for it, secondly that Tempora and the rest of Snowden's observations suggest he might not have described reality.

Asked about data harvesting, the man from GCHQ used the apt-enough analogy of the world as a giant wheat field. He was only interested, he said, in needles and fragments of needles in one haystack in one corner of the field. Everything else - and Lobban was insistent on this point - was left alone. Let's say that's true. It might justify the most intensive search possible of wheat and chaff and lethal needles. But not one member of the committee asked how the haystack was defined, by whom, and by what criteria of threat. That aspect of the Snowden revelations was simply not addressed.

Rifkind and his colleagues would deny it, but they were actors in a playlet designed to show us that there's nothing to worry about, that Britain is in safe hands. The sub-text said, indeed, that worrying and questioning amounted to granting aid and comfort to the enemy. These enemies, in turn, were any kind of horror, potential or actual, you might care to name. The security services are never short of enemies.

But let's be rational. However it came into existence, terrorism is real enough. The 52 who died in the London bombings of July 2007 didn't pick their enemies. They were failed by MI5, not by a lack of Jesuitical quibbling over scrutiny and parliamentary oversight.

But rendition, fake intelligence, allegations of torture, mass surveillance and the sense that liberties are being trampled upon in liberty's name tend to cancel most blank cheques.

On Thursday, the three chiefs were keen to talk about oversight. They answer to ministers, they answer to commissioners (one for the intelligence services, one supervising the interception of communications) and they give answers to Rifkind's committee. But ministers routinely refuse to deal with intelligence matters in public or in the Commons. The commissioners are a pair of judges who meet in secret. Rifkind's team also hears the real stuff, some of it, behind closed doors.

The spooks retort that governments must have secrets and that these cannot be discussed just to satisfy public curiosity. They will discuss every paradox created for democracies, but not why a secret is a kind of metaphysical concept. Ultimately, nevertheless, the agencies demand trust and faith. In effect, they make a demand for censorship, voluntary or imposed. The three talking to Rifkind's people were staunch for freedom, but real press freedom was "irresponsible".

So why should journalists and whistleblowers relent when the likes of the Intelligence and Security Committee does no more than offer loyal support? What about the claim that some secret programmes have been kept from ministers? What about the fact that hundreds of thousands of US spooks have access to those precious British secrets? What happens to UK law when intelligence comes as a gift from America? What happens to the burden of proof when the agencies describe threats?

Amid Rifkind's stage-managed open day, one thing was clear enough. Britain's three main intelligence agencies are not burdened with self-doubt. Nor do their leaders believe that government, parliament or public will impede them in their self-defined tasks. A few terrorist assaults and 34 alleged plots since 2005 have allowed them to expand their empires vastly. And the politicians seem incapable of imposing restraint.

As America has discovered, intelligence has become a trans-global enterprise that does not regard itself as answerable, necessarily, to any particular host state or administration. The defence of the public and a way of life is not an excuse, as such - our trio of chiefs were suitably passionate about their sense of duty - but we have reached the point at which a secret world is setting democracy's limits.

At one point on Thursday, Lobban said: "I don't believe secret means sinister." To his credit, he didn't sound even slightly sinister. His belief is a comfort to him, no doubt, but it is of little help to anyone with a lingering belief in checks and balances. The three men who turned up in Westminster, our guardians, doing their best to exude humility, were more powerful than most people in the building. And that's our problem.