Sir Arthur Conan Doyle was the brilliant author of the wildly popular Sherlock Holmes detective stories, which celebrated the triumph of reason and logic over superstition and magical thinking. Unfortunately, the Scottish physician-turned-writer did not apply his creation’s cognitive skills when it came to the blossoming spiritualism movement of the early 1900s: he fell blindly for the crude hoax of the Cottingley Fairies photographs and regularly attended séances to make contact with family members who had died in the First World War, especially his son Kingsley. Perhaps fittingly, Conan Doyle’s fame brought him into company with the greatest magician of his age, Harry Houdini, who did not suffer fakes gladly.

In the spring of 1922 Conan Doyle visited Houdini in his New York City home, whereupon the magician set out to demonstrate that slate writing—a favorite method among mediums for receiving messages from the dead, who allegedly moved a piece of chalk across a slate—­could be done by perfectly prosaic means. Houdini had Conan Doyle hang a slate from anywhere in the room so that it was free to swing in space. He presented the author with four cork balls, asking him to pick one and cut it open to prove that it had not been altered. He then had Conan Doyle pick another ball and dip it into a well of white ink. While it was soaking, Houdini asked his visitor to go down the street in any direction, take out a piece of paper and pencil, write a question or a sentence, put it back in his pocket and return to the house. Conan Doyle complied, scribbling, “Mene, mene, tekel, upharsin,” a riddle from the Bible’s book of Daniel, meaning, “It has been counted and counted, weighed and divided.”

How appropriate, for what happened next defied explanation, at least in Conan Doyle’s mind. Houdini had him scoop up the ink-soaked ball in a spoon and place it against the slate, where it momentarily stuck before slowly rolling across the face, spelling out “M,” “e,” “n,” “e,” and so forth until the entire phrase was completed, at which point the ball dropped to the ground. According to William Kalush and Larry Sloman in their 2006 biography The Secret Life of Houdini (Atria Books), the Master Mystifier then dealt Conan Doyle the lesson that he—and by implication anyone impressed by such mysteries—needed to hear:

Sir Arthur, I have devoted a lot of time and thought to this illusion ... I won’t tell you how it was done, but I can assure you it was pure trickery. I did it by perfectly normal means. I devised it to show you what can be done along these lines. Now, I beg of you, Sir Arthur, do not jump to the conclusion that certain things you see are necessarily “supernatural,” or the work of “spirits,” just because you cannot explain them....

Lamentably, Sir Arthur continued to believe that Houdini had psychic powers and spiritual connections that he employed in his famous escapes.

This problem is called the argument from ignorance (“it must be true because it has not been proven false”) or sometimes the argument from personal incredulity (“because I cannot imagine a natural explanation, there cannot be one”). Such fallacious reasoning comes up so often in my encounters with believers that I conclude it must be a product of a brain unsatisfied with doubt; as nature abhors a vacuum, so, too, does the brain abhor no explanation. It therefore fills in one, no matter how unlikely. Thus do normal anomalies become paranormal, natural phenomena become supernatural, unidentified flying objects become extraterrestrial spacecraft and chance events become conspiracies.

Houdini’s principle states that just because something is unexplained does not mean that it is paranormal, supernatural, extraterrestrial or conspiratorial. Before you say something is out of this world, first make sure that it is not in this world, for science is grounded in naturalism, not supernaturalism, paranormalism or any other unnecessarily complicated explanations.