The reason he should do this is fairly simple — it is all because of Marian Keech and the messages she started receiving back in the mid-1950s. They came from a mysterious god called Sananda and they told her that a huge flood was coming and was going to swallow up the earth. Only a few would be saved.

The God I pray to is irritatingly vague, but Sananda certainly couldn’t be faulted on that score. He informed Mrs Keech, through high density vibration of her writing hand (how else?), that the whole thing was going down at midnight on December 21.

Within a short time, a small cult had formed around Mrs Keech in her home town of Lake City, Minnesota. They kept their existence pretty quiet because only a few could be saved, but despite this a young psychologist called Leon Festinger became aware of them and began, covertly, to monitor their activities.

It is through Festinger that we know what happened on December 21, when the clock struck midnight and it wasn’t even raining. There they were all sitting, waiting for the spacemen to arrive and whisk them away to the planet Clarion — and nothing. You might have thought they would be at least a little disheartened. Not a bit of it. God had saved the world, they decided, and immediately the secretive group went out and began to publicise their work. Years later they were still heeding Sananda’s warnings.

The experience of Marian Keech led Festinger to write his famous paper A Theory of Cognitive Dissonance. Festinger argued that when ideas and reality collide, people do not adjust their behaviour to suit the newly understood truth. Instead they rationalise, twisting the truth around until it suits their existing behaviour.

What does Festinger’s theory tell us about politics? It tells us that when we complain about politicians getting to power and not doing what they said they would do, we are complaining about the wrong thing. The real problem with politicians is the opposite. Most of the time they do exactly what they promised to do, even if by the time they do it a bit of them knows that it doesn’t make much sense.

And this is why David Cameron should not accept the post of Leader of the Opposition. Indeed, this is why the role should cease to exist.

When a party arrives in government in this country it does so having spent years as Her Majesty’s Loyal Opposition. Almost every waking hour of every senior politician in the party has been spent constructing a case against whatever the government of the day is doing. If this case is not put vigorously enough, particularly in the House of Commons, the politician can expect severe criticism. Shadow Cabinet meetings are consumed with interminable discussions about how best to use Opposition debates to embarrass the other side.

And it is through this work as an opposition that most policy is formed. Sure, a central unit might carve out a few new ideas to adorn an election manifesto, but this accounts for only a small proportion of the party programme. The rest is determined by Shadow Ministers who take positions to help them to attack the government in Parliamentary debates.

Take Labour’s position on grant-maintained schools. When this Government took office, it abolished these schools. Now, with Tony Blair looking at his Swatch nervously because he has to go and give a lucrative lecture in America at any moment, he has suddenly decided that nothing could be more important to his legacy than their reintroduction. They are like, er, well, you know, a really good idea. Why has it taken him eight years to work this out? Because when he pledged to abolish the schools he was opposing the Government, while now he is trying to run it.

In the 1997 Parliament the Conservative party invested a lot of its energy in trying to defeat the privatisation of the National Air Traffic Control System, even though the plans had orginated with the last Tory Government. And now the Tory Opposition is forming its policy on terror legislation mainly in an attempt to discomfit Mr Blair rather than because of a considered policy position.

Even when Opposition work is not corrosive, the best that can be said of it is that it pretty pointless. The routine attacks on the Government convince nobody. When a Tory Shadow pops up on the media attacking Labour, the usual public respsonse is, to paraphrase Mandy Rice-Davis: “They would say that, wouldn’t they?”

Thus when media columnists claim they are better at opposing the Government than the official Opposition they are quite right. But when they go on to moan about this, they are quite wrong. It is far better for the country if the media provide the Government with an opposition while the minority party provides something better to vote for.

So if Mr Cameron becomes Leader of the Conservative Party he should declare that he does not intend to be the Leader of the Opposition. Instead, he should announce that he wants to be called Shadow Prime Minister and that his party will be known as Her Majesty’s Alternative Government.

It may sound like a purely cosmetic change, but it could make a profound difference to the way the Tories behave. In the Commons, amendments to government Bills would be tabled with a genuine desire to improve the legislation and questions would be asked in order to elicit useful answers. In the media, spokesmen for the Alternative Government would concentrate on their own ideas rather than tiresome attacks on proposals that they would probably be making themselves if they were in office.

David Cameron says he’s going to do things differently. Go on then.

daniel.finkelstein@thetimes.co.uk

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