|The Hound from Hell|
The famous quote from "The Adventure of the Sussex Vampire" sets the boundaries clearly enough: The solutions of the canonical Sherlolock Holmes cases are to be found in human beings, not supernatural ones. (Pastiches are a different story.)"This agency stands flat-footed upon the ground, and there it must remain," Sherlock Holmes insisted. "The world is big enough for us. No ghosts need apply."
That's appropriate for a traditional detective story. As Monsignor Ronald A. Knox wrote in his "Ten Rules of Detective Ficiton," "All supernaural or preternatural agencies are ruled out as a matter of course."
But raising the specter - so to speak - of the supernatural and then explaining it later by natural means is perfectly legitimate in a detective story. "The Adventure of the Sussex Vampire" is a perfect example. So is "The Adventure of the Devil's Foot," in which Mortimer Tregennis cries, "It is devilish, Mr. Holmes, devilish! It is not of this world. Something has come into that room which has dashed the reason from their minds. What human contrivance could do that?"
The most famous Holmes story evoking the supernatural only to dispel it is, of course, the highly gothic Hound of the Baskervilles. Who can ever forget those chilling words "Mr. Holmes, they were the footprints of a gigantic hound"?
Almost every mystery writer sooner or later writes a story of a crime that appears to admit of no earth-bound explanation. John Dickson Carr did it again and again, both under his own name and that of Carter Dickson.
But did anyone do it before Arthur Conan Doyle? There were plenty of gothic novels in the pre-Holmes period, and plenty of detective stories, but were there any pure fair-play detective stories with an air of the supernatural? Or was this an innovation for which the Master has not received due credit?