The currents and eddies of right and wrong, which you find such plain sailing, I cannot navigate. I'm no voyager. But in the thicket of the law, oh, there I'm a forester. I doubt if there's a man alive who could follow me there, thank God.

“This country's planted thick with laws from coast to coast - man's laws, not God's - and if you cut them down - and you're just the man to do it - d'you really think you could stand upright in the winds that would blow then? Yes, I'd give the Devil benefit of law, for my own safety's sake.”

Are there wiser words in literature than those Robert Bolt put in the mouth of Sir Thomas More in A Man for All Seasons? Perhaps there are more exquisite phrases to be found on the nature of love or more descriptive passages on the horror of war. But when it comes to politics and statesmanship, surely there are none finer.

This morning I feel the weight of Bolt's words because we have been tested, you see. And we have come through.

Was there ever anything more preposterous, more manipulative, more laughably deranged than Mohamed Al Fayed's allegation that the Duke of Edinburgh had Princess Diana murdered? Weren't the lying and the bullying obnoxious? Hasn't the whole three-ring circus been nauseating?

Yet still we have pressed on. Still, we have insisted that every man - even this man - must have his day in court. Still, even under this intense provocation, we have shown that we know that we cannot navigate right and wrong but merely be foresters in the thickets of the law.

The Diana inquest cost a fortune. The verdict will not change the mind of a single person. The conclusions the court reached were entirely obvious to anyone with half a brain right from the beginning. But still. Worth every penny, it was. But still. We should be proud not ashamed.

Of course, the inquest will not put the conspiracy theories to rest. Some people will continue to believe that the Princess was killed, and that one of her assassins drove her Mercedes into a pillar using his not-very-James-Bond Fiat Uno. Oh yes, and for some as yet unexplained reason, did so with his dog in the back of the car. I would have left my pet at home before setting off to kill a member of the Royal Family, but what do I know about being a spy?

Such theories are pretty much immune to reason. They are the product of a certain mindset - endlessly suspicious, always blaming “them” for what happens to “us” - and a failure to understand basic probabilities.

Conspiracy theorists make an elementary error. They see the incredible coincidences, the startlingly improbable facts, in any terrible death - Martin Luther King, the Kennedys, John Lennon, Diana - and believe they could not have just happened. There must have been a reason. But this is quite wrong. While the chances of any one particularly striking event happening may be vanishingly small, the number of improbable events that conceivably could happen is almost infinite. So the overall probability of a startling event or combination of events of some kind is quite high, as long as you aren't specific in advance about what it is.

Take the lottery. The chances that you will win the lottery this week are pitifully small. But the chance that someone, somewhere, will win the lottery is very high. If I identified you as this week's winner in advance (provided I didn't do that every week) and you then won, that would be extremely surprising. If you simply won, that would be amazing for you, but not for anyone else.

Proceeding from this mathematical mistake, the theorists start to pull at every loose thread, certain that the whole conspiracy will unravel if they pull hard enough. And they go on pulling, however little progress they are making. You can no longer be an expert in the conspiracy to kill John F. Kennedy. The literature is too big to master. You have to specialise - the magic bullet, the autopsy, the false Oswalds, the cover-up.

One day perhaps we'll have specialists like that on the death of the Princess - the Fiat, the dog, the Burrell letters, the engagement. I can just see it, can't you?

So if they are beyond persuasion, why did we bother? Because even if they don't care for truth, we do. And even though our ability to identify the truth is deficient, a court of law is our best hope.

Whatever one might think of conspiracy theorists and their borderline mad obsessions, we cannot blithely assume that conspiracies never exist. Even quite bizarre ones.

François Mitterrand conspired with a far-right lunatic to fake an attempt on his own life hoping that the shave with death would win him political sympathy; Richard Nixon's election campaign bugged opponents and forged letters in their name to discredit them; Lyndon Johnson fixed the ballot that won him a seat in the Senate; Harold Wilson's doctor canvassed the ideas of murdering the Prime Minister's political secretary; British intelligence agents based in Latvia concocted communist correspondence to discredit the first Labour Government; communists infiltrated the US Government in the 1940s and '50s and stole sensitive documents.

I would rather live in a society that took mad theories too seriously than in one that failed to take wrongdoing seriously enough.

The law is cumbersome, it is clumsy, but an ass? No.

daniel.finkelstein@thetimes.co.uk

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