The second commandment of mystery fiction is:
All supernatural or preternatural agencies are ruled out as a matter of course.
Father Knox’s 1939 commentary on this was:
To solve a detective problem by such means would be like winning a race on the river by the use of a concealed motor-engine. And here I venture to think there is a limitation about Mr. Chesterton’s Father Brown stories. He nearly always tries to put us off the scent by suggesting that the crime must have been done by magic; and we know that he is too good a sportsman to fall back upon such a solution. Consequently, although we seldom guess the answer to his riddles, we usually miss the thrill of having suspected the wrong person.
When it comes to committing the murder, I think that this is a fairly non-controversial rule. If there are ghosts who can kill people in the story, then one of two possibilities must be the case:
- We know about this from the beginning.
- We find out about it later.
If #1 is the case, there is no mystery to a locked-room mystery, or really any murder in the haunted house at all. If #2, this violates the first commandment, since we will not have met the murderer early on in the story.
There is the further problem that the demands of justice do not obtain. If there are spirits who are free to kill the living wandering around, they do not have any particular obligation to us to leave us alive. And as a matter of practice, the solution to the mystery is entirely academic, since one cannot put a ghost in prison.
Similarly, the detective receiving the solution to the problem from omniscient or clairvoyant beings is simply not interesting as an intellectual puzzle. One can try to match wits with the detective, but not with God. And really, there’s no difference between between having a ghost tell the detective the solution to the problem and having a living witness to the crime. And no one would write a murder mystery in which the solution was someone who saw the murder came and told the solution to the detective. After all, in that case, he’s at best a stenographer, not a detective.
There is, however, an exception to the rule, if one wants to put a book simultaneously into both mystery and paranormal genres—it would work to have a ghost be the client of the detective. There are some limits to this, of course—it would be hard to have the ghost unaware of who stabbed him to death, for example. But it could certainly work to have the ghost want to know who poisoned him, or who set a trap which killed him, etc. It would offer a fair amount of leeway, too, for how paranormal one wanted the story to be. The ghost could have no interaction with the detective other than kicking off the mystery, in which case it could be questionable whether there even is a ghost. On the other end of the spectrum, the ghost could become the side-kick of the detective, taking advantage of his ability to walk through walls to do detection which is too dangerous or simply unavailable to the detective, but the detective providing the brains of the operation. As a matter of personal taste, it’s not the sort of story I want to write, but it’s definitely doable.
Though it’s not about the command itself, I think a work is in order about Fr. Knox’s commentary on Chesterton’s Father Brown mysteries. I think he slightly mischaracterizes what Chesterton was doing when characters in the story would suggest magic or ghosts as the solution. I do not think that Chesterton was trying to fool the reader, any more than Scooby Doo was trying to fool the audience. Rather, I think Chesterton was setting an atmosphere.
There are two environments in which a detective can operate: one of ignorance, and one of confusion. The people can know nothing, and turn to the detective, or the people can be mistaken, and need the detective to correct them. Many of the classic detective stories are the former. A body is found, there are no witnesses and no fingerprints—who did it? But there is the other sort, where people are sure that someone did it, and the detective must prove them wrong. And this is what Chesterton was setting up.
There is also simply the fun of the thing. Chesterton loved to play with the stereotype of faith-vs-reason and the hard-headed skeptic vs. the credulous priest. And it’s fun because the stereotype is wrong. It is, generally, the skeptics who are credulous. And quite often, it should be added, the priests who are skeptical.
This may be best summed up in one of my favorite passages from the Father Brown mysteries, specifically in the first, The Blue Cross. (To give context, Flambeau is the arch criminal who tried to steal the eponymous blue cross from Fr. Brown but was foiled, and Fr. Brown explained various thieves’s tricks which to defend the blue cross he employed or was prepared for Flambeau to try, some of which even Flambeau hadn’t heard of.)
“How in blazes do you know all these horrors?” cried Flambeau.
The shadow of a smile crossed the round, simple face of his clerical opponent.
“Oh, by being a celibate simpleton, I suppose,” he said. “Has it never struck you that a man who does next to nothing but hear men’s real sins is not likely to be wholly unaware of human evil?”
If you like murder mysteries, you might like murder mystery, The Dean Died Over Winter Break.