Monsignor Ronald Knox was an avid reader, critic and essayist on mystery fiction in the early part of the Twentieth Century, as well as a novelist and writer of short stories. Originally an Anglican, he converted to Catholicism in 1917. In 1928 he published his “Decalogue of the Mystery: The ten rules of detective fiction.” Some of the rules are written in language that today is offensive, but I have tried to preserve the original, not as a comment on the writer, but as a comment on the society of the time.
1. Introduce the murderer early, but readers should not be allowed to know the murderer’s thoughts.
2. All super natural or preternatural agencies are to be ruled out.
3. No accidents or unaccountable intuition.
4. Only one secret passageway is allowed.
5. All clues must be shown at once.
6. Never make the detective the killer.
7. No exotic rule-free killers. (actually he wrote no Chinamen)
8. No undiscovered poisons.
9. No unprepared-for twins or doubles.
10. The stupid friend of the detective must never conceal any thoughts which pass through his mind, and his intelligence must be very slightly below that of the average reader.
These ten rules have guided writers ever since. Of course, many fine writers have and continue to twist, bend and violate those rules. But, as Beethoven is purported to have said to one of his students of composition, “it is hard to effectively break the rules until you know what the rules are.”