Of course, at the time Blair said this he was peddling some pretty absurd conspiracy theories himself, so perhaps we should consider him an expert.

When most of us think of conspiracy theories we're thinking of faked moon landings and government cover-ups of encounters with aliens. However, when the political classes use the term they also mean any interpretation of a government's actions that deviates from the narrative they've helpfully set out for us. Here the implied lunacy of the "conspiracy theorist" is to suggest that our leaders might be less than morally pristine; that they're not basically just trying to "do the right thing", whatever mistakes might be made along the way.

Let's briefly recall some of the absurd conspiracy theories and other crackpot notions that our Prime Minister, along with others, has tried to convince us of. One was that Saddam Hussein - the tinpot dictator of a crippled third world country that had been smashed by a decade of war, sanctions and bombing - posed a dangerous threat to the greatest superpower the world had ever seen. The story went that nations with enough weaponry to end life on earth had something serious to fear from one of the weakest and most isolated countries in existence. No plausible explanation was given as to why this one-legged pygmy would dream of launching himself at a herd of elephants, let alone a herd of elephants armed with tactical nuclear weapons. In fact there was serious doubt that the pygmy had so much as a bow and arrow or a vial of anthrax to wave unconvincingly at his trembling victims before he got trampled in the stampede. But sober and sensible commentators, pundits and politicians mulled over this palpable claptrap for months before finally the biggest elephant of all went ahead and impaled the hapless pygmy on its right tusk; just to be safe.

Another half-baked notion that somehow grips the imaginations of opinion-makers is that a deep concern for the plight of the Iraqi people helped to inspire the US and the UK to topple the monstrous Saddam. In fact to describe the idea as 'half-baked' is to inflict a mortal insult on slapdash bakers everywhere. Yet the political classes have doggedly clung to this conviction despite the fact that the US and the UK backed Saddam while he committed all his worst atrocities and maintained a sanctions regime upon Iraq in the 1990s that UN officials described as genocidal, which killed over a million civilians; 4,000 under-fives per month according to UNICEF. Not exactly the behaviour of governments driven by the sheer moral urgency of saving the Iraqis from torment. But then no self-respecting conspiracy theorist is going to let themselves be confused by the facts.

Then there was the theory that Saddam and al-Queda had joined forces, much as The Riddler and The Penguin had joined forces in Batman the Movie (starring Adam West). Again the inconvenient facts - like the mutual enmity of the bad guys, al-Qaeda's wish to overthrow Saddam, Saddam's persecution of radical Muslims - were not enough to shoot down these flights of fancy.

Sadly, to look at the question of oil in relation to Iraq, we must return to a more mundane view of the world, but one that at least has the redeeming feature of being sane.

If one were to say that the shareholders of a major corporation sought to maximise profit and market share, that would not be a conspiracy theory so much as a bald statement of fact. Similarly, if one were to say that great powers throughout history have sought both access to and control over essential resources in order to enhance their power, that would hardly be a controversial statement. So it should have surprised no one when renowned investigative reporter Greg Palast, reporting last week for BBC Newsnight, revealed details of the tactical squabbles going back to 2001 that had taken place within US state and corporate circles over the post-invasion fate of Iraqi oil. No more than it should be a surprise to learn that the US State Department, in 1945, described Middle Eastern oil as "a stupendous source of strategic power, and one of the greatest material prizes in world history". Or that British planners in 1947 concurred, describing it as "a vital prize for any power interested in world influence or domination". Or that in 1956 Foreign Secretary Selwyn Lloyd had noted "We must at all costs maintain control of this oil". Or that in 1999 Dick Cheney, now US Vice-President, told the Institute for Petroleum that "Oil is unique in that it is so strategic in nature. We are not talking about soapflakes or leisurewear here. Energy is truly fundamental to the world's economy. The Gulf War was a reflection of that reality".

The proposition that Iraq was invaded primarily because of its oil can hardly be described as a conspiracy theory since, unlike flimsy notions of a "WMD threat" or "humanitarian intervention", it is based on rational observations concerning matters of fact. But the reason that the Prime Minister and others try to push such talk to the margins of debate has less to do with rationality than with the need to make room for the self-serving chauvinistic slogans that politicians and their apologists must proclaim in order to justify warfare. That a decision which may have caused the deaths of over 100,000 civilians can be discussed with such casual disregard for the most basic facts should give you the true measure of our political culture.