Ronald Knox: 10 Commandments of Detective Fiction-- Which do you argue with?

Ronald Knox: 10 Commandments of Detective Fiction

Ronald Knox was quite the eclectic fella-- a Roman Catholic priest (so of course he has to compile 10 Commandments), as well as a mystery writer who hung out with Agatha Christie and GK Chesterton in the Detection Club.  He came up with these rules for mysteries, some of which (#5?) are kinda obsolete

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 allowable.
4. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.
5. No Chinese man 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.
10. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.

 Well, these are fun, anyway! I have another list of TWENTY I'll post later.