Hitler did not die in the Berlin bunker. Disguised as SS troops, American special forces, using the nearby Unter den Linden as an airstrip, had spirited him away just minutes before the Russians arrived. He died in 1959 in a suburban bungalow in Wichita. Unaware of his true identity, many employees from the local motorboat factory where he had worked as a storeman attended his funeral.

This is an outline of a story told to me in a cafe by a Dutchman a few years ago. It was a long and detailed story. I asked the Dutchman how he came into such information. He leaned forward and all but tapping the side of his nose whispered that a Red Army colonel had told him.

Normally, conspiracy theorists are easily spotted and consequently easily avoided. But the Dutchman was different. He had a responsible government job, and he talked and looked like a normal person up to the point where he started on the Hitler story. He was a stealth conspiracy theorist.

After a while of listening to the Dutchman and trying to be polite, I could no longer suppress the urge to burst out laughing, which I duly did, in the process spraying his suit with a mouthful of half-chewed frikandel.

I used to worry about the unintended rudeness of this event, for I used to think that while conspiracy theorists are just tedious attention-seekers, they are also rather pitiable souls. But the deadly MMR fraud has since sharpened my thinking. Now I am inclined to think that conspiracy theorists cannot be laughed at enough. While it may be true that most of their beliefs are little more than harmless self-aggrandisement, they are nevertheless all from the same stable and all can be used as a front for other prejudices and hatreds: the MMR fraud (anti-science), the birther movement (racism) and the blood libel (anti-semitism), the Dutchman (anti-Americanism) to name but a few.
It might have been better if the accident with the frikandel had resulted in it going straight into the Dutchman's face, an outcome which, as it turned out, the other Dutch people round the table would have preferred.

There are those who say we should seek to change the beliefs of conspiracy theorists by talking to them with facts and rational thinking. Anyone who thinks that has surely never tried it. All you get back is banal stuff like, 'Ah, but how do you know there is no Loch Ness monster?!' – the exclamation mark illustrating the triumph of what they imagine to be a knock-down argument. Or they hurry away to consult their computers or the higher number Freeview channels in order to find the killer fact that will settle your hash: 'See! Sir Mark Rylance says Shakespeare didn't write Shakespeare and he should know because he's an actor!'

Referring to these people as theorists devalues the notion of a theory. A theory is a device built from relatively few known facts in order to predict a larger number of as yet unknown facts. Once predicted, they can then be tested. If they are found, then the theory is supported. If not, then the theory is not supported. In this way, theories either develop and grow, or they wither away, sometimes ultimately to be junked. Theories are good. They have given us everything from toasters to aircraft to antibiotics.

Views such as the Dutchman's are not theories. They are built on beliefs that have no referents in reality, or at best misunderstood or concocted referents. Their reasoning, such as it is, is often circular so that they already contain answers to any questions they might raise. Consequently, they often cannot be tested, and since they cannot be tested they are not theories.

Insofar are their devotees do test them, they do so by means of verification. That is to say, like those dashing off to find the killer fact, they look only for that. It never occurs to them that it is possible to verify anything. A belief that red-haired men have big ears is easily verified simply by finding some such men. The pseudo-scientists would never think of looking for instances of red-haired men with normal-sized ears.

Looking for counter instances, or falsification, is how science makes progress. The distinction between the two is not exclusive to science. It has its parallels in everyday life. The truth of beliefs can easily be confirmed simply by attending only to information that reflects what their disciples already think. When a counter instance unavoidably comes up, it can be dismissed as nonsense and its purveyor as narrow-minded.

For example, The Guardian has described the J B Priestley play, An Inspector Calls, as 'thrilling and pertinent'. The Spectator has described it as 'poisonous, revisionist propaganda, hysterical tosh'. If a person is attached to one of these views, the most likely effect of the other view would be to make them even more attached to it. Thus, views are satisfyingly and repeatedly confirmed. But they remain static, except for being ever more emphatically expressed as the supporting evidence of verification accumulates.

It would be good to think that conspiracy believers are nothing more than irksome pub bores. And most of them are just that, sitting there, not often enough plastered in frikandel. But others are altogether more sinister, whether or not they know it.