Why do people believe weird things? The easy answer is: because they are dumb or, more politely, irrational. But one doesn’t need to be particularly intelligent to avoid weird beliefs. And weird beliefs can be upheld by very intelligent people. Why? That is the hard question. An answer to that question can easily encompass simpler forms of irrationality.
A great case in point is that of Sir Arthur Conan Doyle, the creator of rationality personified, Sherlock Holmes. Here he is in a 1927 film:
The first half is a delightful account of how Sherlock Holmes came about – the idea of creating a detective who would build up conclusions based on scientific methods and the power of observation. But even more fascinating is the second part – a mindboggling account of Sir Arthur’s lifelong advocacy of Spiritualism.
What was going on? Had Sherlock’s papa gone gaga? No. Notice what he said at 7:50:
When I talk on this subject, I am not talking about what I believe, I am not talking about what I think – I am talking about what I know… I am talking about things I handled, I have seen, that I have heard with my own ears, and always, mind you, in the presence of witnesses – I never risk hallucination.
Definitely holmesque. In our framework, Sir Arthur was not propounding a Faith: BR=1, irrespective of evidence. He was proclaiming a Certainty, entirely grounded on what he portrayed as conclusive evidence. Beliefs are the result of a tug of war between confirmative and disconfirmative evidence, where evidence accumulates multiplicatively:
In evaluating the hypothesis: Spiritualism is true, Sir Arthur relied entirely on evidence. He did not say that evidence was uniformly, or even preponderantly, in favour of the hypothesis. He was well aware of numerous fakes and swindlers. What he said was that he had witnessed experiments – many, as it turned out – that proved the reality of psychic phenomena, and did so incontrovertibly. In our language, he had seen Smoking Guns: FPR=0, that had driven his Posterior Odds all the way up to infinity. As a result, he was certain that Spiritualism was true.
So the hard question becomes: how could Sir Arthur possibly view those experiments as conclusive evidence? Here is where Sherlock Holmes fits in. In the world of fiction, conclusive evidence is what typically brings out a complete resolution of uncertainty. In the end, we all know whodunit. Sherlock Holmes is the maestro of conclusive evidence. In his memorable words: “When you have eliminated the impossible, whatever remains, however improbable, must be the truth” (The Sign of Four, Chapter 6). But Holmes could rely on conclusive evidence because his author fabricated it for him. In the real world, evidence is not as neatly conclusive, and the impossible is harder to eliminate. It was Sir Arthur’s penchant for Smoking Guns, nurtured by his creative talent, that lead him to see them once too often. He was blinded by evidence.
This fascination with conclusiveness brought him to paradoxical, even comical extremes. The first chapter of his last book, The Edge of the Unknown, published in 1930, the year of his death, is devoted to The Riddle of Houdini:
I have never met a man who had such strange contrasts in his nature, and whose actions and motives it was more difficult to foresee or to reconcile (p. 1).
Quite. Another great example of Omelette Flipping. Sir Arthur and Harry Houdini had a long and tempestuous relationship.
Conan Doyle was convinced that the great magician was endowed with supernatural powers, and obstinately tried to persuade him to embrace the Spiritualist cause. But Houdini was in fact an adamant opponent of Spiritualism. His relationship with Conan Doyle, well depicted in chapter 20 of the Kalush and Sloman biography, was mainly motivated by the access that, through him, he could gain to those experiments that, while so much impressing Sir Arthur, Houdini could readily expose as hoaxes and frauds. Wary of causing offence, Houdini gently cautioned Sir Arthur against eliminating alternative explanations. But regarding his own feats, he explicitly told him, and everybody else, that they were based on extraordinary but entirely natural skills. Sir Arthur, however, would have none of it. So, after Houdini died in 1926, he set out to explain his Riddle:
Once the mind is adjusted to the false assumption that psychic powers do not exist, then all reasoning power seems to become atrophied, as is the case in all bigoted religions. As an example it was said, and is said, again and again, “How absurd for Doyle to attribute possible psychic powers to a man who himself denies them!” Is it not perfectly evident that if he did not deny them his occupation would have been gone forever? (The Edge of the Unknown, p. 15).
Amazing. He is saying: if the denial of Spiritualism becomes an article of Faith (BR=0), no amount of evidence can change it. That is true. But so it is if BR=1, which occurs when the iteration of probability updates encounters conclusive evidence that proves the irrefutable truth of the hypothesis. But what is conclusive evidence? Here is a taste, according to Sir Arthur:
Inexperienced and foolish people may jeer, but they will find it easier to do so than to refute the evidence. For example, upon June 3rd, 1871, Mrs. Guppy was floated from her own house in Highbury, and appeared upon the table of a room at 61 Lambs Conduit Street, where a séance was being held behind locked doors. A document was signed by the eleven sitters to testify to the fact and they had no possible object in perjuring themselves about the matter. Mrs. Guppy said that the last thing she could remember was sitting with her friend Miss Neyland. That lady deposed that Mrs. Guppy has suddenly vanished from her sight. Four of the sitters accompanied Mrs. Guppy home and heard what her friend had to say. It is difficult to find any flaw in such evidence and it would certainly have been conclusive in a court of law had it been a criminal case (p. 24).
Oh well. Remember Sherlock: When you have eliminated the impossible…
Sir Arthur was definitely not as clever as his creature at detecting authentic conclusive evidence. But then he lived in a much more complicated world: the real one. This cavalier attitude to conclusiveness is typical of weird believers. A more recent example is that of 9/11 truthers, who think that the hypothesis that the planes caused the collapse of the twin towers is not only improbable: it is impossible.
Extraordinary claims require extraordinary evidence. Or, in the words of the compère of Bayes’ Theorem, Pierre-Simon Laplace:
The more extraordinary the event, the greater the need of its being supported by strong proofs. For those who attest it, being able to deceive or to have been deceived, these two causes are as much more probable as the reality of the event is less (A Philosophical Essay on Probabilities, p. 17).
No hypothesis should be accepted or rejected outright. It is good to keep an open mind even to the weirdest hypothesis – where weird means: in contrast with common experience. But the weirder the hypothesis, the stronger the required evidence, where strength refers not to our level of conviction, but to the rigorousness of the process through which the hypothesis is vetted.
That includes the Supernatural. Famously, since 1964 Harry Houdini’s reincarnation, James Randi, has pledged to award one million dollars to anyone who can demonstrate any kind of supernatural power. Randi is saying: according to my experience, the probability of the Supernatural is very close to, but not equal to zero. I am prepared to change my mind. But I will do so only under conditions that I believe will provide me with conclusive evidence that the hypothesis is true. Unsurprisingly, the million dollar is still there.
If anything supernatural titillates your curiosity, have a look at Randi’s Encyclopedia before jumping to conclusions.