Jorge Luis Borges on the rules of detective fiction
A.) A discretional limit of six characters. The reckless infraction of this law is responsible for the confusion and tedium of all detective movies. In every one we are presented with fifteen strangers, and it is finally revealed that the evil one is not Alpha, who was looking through the keyhole, nor Beta, who hid the money, nor the disturbing Gamma, who would sob in the corners of the hallway, but rather that surly young Upsilon, whom we’d been confusing with Phi, who bears such a striking resemblance to Tau, the substitute elevator operator. The astonishment that this fact tends to produce is somewhat moderate.
B.) The declaration of all the terms of the problem. If my memory (or lack of it) serves me, the varied infraction of this second law is the favorite defect of Conan Doyle. It involves, at times, a few particles of ashes, gathered behind the reader’s back by the privileged Holmes, and only derivable from a cigar made in Burma, which is sold in only one store, which is patronized by only one customer. At other times, the cheating is more serious. It involves a guilty party, horribly unmasked at the last moment, who turns out to be a stranger, an insipid and torpid interpolation. In honest stories, the criminal is one of the characters present from the beginning.
C.) An avaricious economy of means. The final discovery that two characters in the plot are the same person may be appealing—as long as the instrument of change turns out to be not a false beard or an Italian accent, but different names and circumstances. The less delightful version—two individuals who imitate a third and thus provide him with ubiquity—runs the certain risk of heavy weather.
D.) The priority of how over who. The amateurs I excoriated in section A are partial to the story of a jewel placed within the reach of fifteen men—that is, of fifteen names, because we know nothing about their characters—which then disappears into the heavy fist of one of them. They imagine that the act of ascertaining to which name the fist belongs is of considerable interest.
E.) A reticence concerning death. Homer could relate that a sword severed the hand of Hypsenor and that the bloody hand rolled over the ground and that blood-red death and cruel fate seized his eyes; such displays are inappropriate in the detective story, whose glacial muses are hygiene, fallacy, and order.
F.) A solution that is both necessary and marvelous. The former establishes that the problem is a “determined” one, with only one solution. The latter requires that the solution be something that the reader marvels over—without, of course, resorting to the supernatural, whose use in this genre of fiction is slothful and felonious. Also prohibited are hypnotism, telepathic hallucinations, portents, elixirs with unknown effects, ingenious pseudoscientific tricks, and lucky charms. Chesterton always performs a tour de force by proposing a supernatural explanation and then replacing it, losing nothing, with one from this world.