The ultimate conspiracy theory goes something like this: "He who questions the truth of the conspiracy is, ipso facto, part of it". It is a wonderfully circular argument - the antithesis of argument, in reality, in that those who subscribe to such a view are not interested in argument, debate or evidence that runs contrary to their beliefs. And those who are sceptical are, quite obviously, fully paid up members of the conspiracy. It is a cul-de-sac perspective from which there is no escape. It provides conspiracy theorists with a comfort blanket and a never -ending validation of what they believe but cannot prove.
Thus those who hold that Diana, Princess of Wales, was murdered in Paris nine years ago are unlikely to be swayed by yesterday's publication of the 832-page report by Lord Stevens of his investigation into the crash that killed her. The circumstances surrounding her death have generated more conspiracy theories than almost any other event in recent decades.
The central role, in this regard, has been played by a grieving Mohamed al Fayed, the father of Dodi Fayed, who died with the princess, along with their driver, Henri Paul. From soon after the crash, Mr al Fayed has alleged variously that members of the British royal family, together with Britain's foreign intelligence service, MI6, were behind Diana's death. In essence, the conspiracy theory is that the British establishment was unwilling to accept the prospect that she was about to marry a muslim and indeed was already pregnant by Dodi. The establishment, it was suggested, would not allow the future king of England to have a half brother or sister who was a muslim.
No evidence has emerged to support these allegations (least of all that Diana was pregnant) but they have gained some currency in a world more sceptical than heretofore. More poisonously, they are believed widely in the muslim world where they are seen to validate negative views of the west. But there is every reason to accept Lord Stevens and his detailed report.
As deputy chief constable of Cambridgeshire, he was fearless when probing matters that were threatening to the British establishment - allegations of collusion between members of the Royal Ulster Constabulary, the British army and members of Britain's domestic intelligence service, MI5. And as deputy chief constable of London's Metropolitan Police, and later its chief constable, he made his name fighting corruption.
His investigation has supported a simple truth: that Diana and Dodi died because of a car crash involving a drunk driver, excessive speed and the paparazzi.