The "materialisation", meanwhile, was accomplished not by ectoplasm, but by another secret assistant. An acrobat, dressed entirely in black tights, scaled the building and entered through a window after the committee had completed their search of the room. According to the magicians, "the ghost" was a bit of gauze coated with phosphorescent paint that the acrobat removed from their pocket and waved around the room.

To confirm their point, the illusionists repeated their demonstration before an even larger committee. And this is where Conan Doyle's reaction gets particularly interesting – he doubled down on his mistaken belief. Not only did he insist that what he had seen in the first séance was not what the magicians subsequently described, but he added that even if the second demonstration was accomplished by a trick, "there is nothing to show that the first séance was not genuine". He was sceptical of the sceptical explanation.

Perhaps the magicians really were psychic, he said, but were simply lying about their paranormal powers. He went on to rail against the media coverage of the demonstration. "It is time which will prove our cause," he wrote. "Time will also prove to those who have misrepresented us that they are playing with fire. They are not judging the Unseen. The Unseen is judging them." Reading those words now, I was struck that his sentiments, if not his coherent sentence structure, would not be out of place on a 21st Century Twitter feed.

Thinking about thinking

In a later TV interview, Conan Doyle tried to explain his views: "When I talk on this subject I'm not talking about what I believe. I'm not talking about what I think. I'm talking about what I know. There's an enormous difference, believe me, between believing a thing and knowing a thing, and talking about things that I've handled, that I've seen, that I've heard with my own ears. And always mind you in the presence of witnesses. I never risk hallucinations. I usually, in most of my experiments, have had six, eight, or 10 witnesses, all of whom have seen and heard the same things that I have done."

Given this reasoning, it is easy to take a dim view of Conan Doyle's own scientific detective skills, but I do think that he was genuinely a brilliant man. In addition to his literary accomplishments, he also worked as a real-life legal advocate, using Sherlock-esque techniques to exonerate and free several falsely convicted prisoners.

Conan Doyle’s reactions to these hoaxes are clearly problematic, but they are also an illustration of psychological phenomena known as "metacognitive illusions".