Detective fiction rules

Thanks to the lovely Tim Symonds, who sent us this link, we now give to you an interesting list - Monsignor Ronald Knox's 10 rules of detective fiction (circa 1928-29):

1. The criminal must be someone mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to follow;
2. All supernatural or preternatural agencies are ruled out as a matter of course;
3. Not more than one secret room or passage is allowed;
4. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end;
5. No Chinaman must figure in the story;
6. No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right;
7. The detective must not himself commit the crime;
8. The detective must not light on any clues which are not instantly produced for the inspection of the reader;
9. The stupid friend of the detective, the Watson, must not conceal any thoughts which pass through his mind; his intelligence must be slightly, but very slightly, below that of the average reader; and
10. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.