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Knox's 1st
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. The mysterious stranger who turns up from nowhere in particular, from a ship as often as not, whose existence the reader had no means of suspecting from the outset, spoils the play altogether. The second half of the rule is more difficult to state precisely, especially in view of some remarkable performances by Mrs. Christie. It would be more exact to say that the author must not imply an attitude of mystification in the character who turns out to be the criminal.

Knox's 2nd
All supernatural or preternatural agencies are ruled out as a matter of course. 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.

Knox's 3rd
No more than one secret room or passage is allowable. I would add that a secret passage should not be brought in at all unless the action takes place in the kind of house where such devices might be expected. When I introduced one into a book myself, I was careful to point out beforehand that the house had belonged to Catholics in penal times. Mr. Milne's secret passage in the Red House Mystery is hardly fair; if a modern house were so equipped - and it would be villainously expensive - all the countryside would be quite certain to know about it.

Knox's 4th
No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end. There may be undiscovered poisons with quite unexpected reactions on the human system, but they have not been discovered yet, and until they are they must not be utilized in fiction; it is not cricket. Nearly all the cases of Dr. Thorndyke, as recorded by Mr. Austin Freeman, have the minor medical blemish; you have to go through a long science lecture at the end of the story in order to understand how clever the mystery was.

Knox's 5th
No Chinaman must figure in the story. Why this should be so I do not know, unless we can find a reason for it in our western habit of assuming that the Celestial is over - equipped in the matter of brains, and under - equipped in the matter of morals. I only offer it as a fact of observation that, if you are turning over the pages of a book and come across some mention of 'the slit - like eyes of Chin Loo', you had best put it down at once; it is bad. The only exception which occurs to my mind - there are probably others - is Lord Ernest Hamilton's Four Tragedies of Memworth.

Knox's 6th
No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right. That is perhaps too strongly stated; it is legitimate for the detective to have inspirations which he afterwards verifies, before he acts on them, by genuine investigation. And again, he will naturally have moments of clear vision, in which the bearings of the observations hitherto made will become suddenly evident to him. But he must not be allowed, for example, to look for the lost will in the works of the grandfather clock because an unaccountable instinct tells him that that is the right place to search. He must look there because he realizes that that is where he would have hidden it himself if he had been in the criminal's place. And in general it should be observed that every detail of his thought - process, not merely the main outline of it, should be conscientiously audited when the explanation comes along at the end.

Knox's 7th
The detective must not himself commit the crime. This applies only where the author personally vouches for the statement that the detective is a detective; a criminal may legitimately dress up as a detective, as in the Secret of Chimneys, and delude the other actors in the story with forged references.

Knox's 8th
The detective must not light on any clues are not instantly produced for the inspection of the reader. Any writer can make a mystery by telling us that at this point the great Picklock Holes suddenly bent down and picked up from the ground an object which he refused to let his friend see. He whispers 'Ha!' and his face grows grave - all that is illegitimate mystery - making. The skill of the detective author consists in being able to produce his clues and flourish them defiantly in our faces: 'There!' he says, 'what do you make of that?' and we make nothing.

Knox's 9th
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. This is a rule of perfection; it is not of the esse of the detective story to have a Watson at all. But if he does exist, he exists for the purpose of letting the reader have a sparring partner, as it were, against whom he can pit his brains. 'I may have been a fool,' he says to himself as he puts the book down, 'but at least I wasn't such a doddering fool as poor old Watson.'

Knox's 10th
Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them. The dodge is too easy, and the supposition too improbable. I would add as a rider, that no criminal should be credited with exceptional powers of disguise unless we have had fair warning that he or she was accustomed to making up for the stage. How admirably is this indicated, for example, in Trent's Last Case!