Fair play is a promise a certain kind of mystery makes to its reader. It is simple to state and hard to keep. It says that the reader has exactly the same evidence as the detective, at the same time she gets it, and that a careful reader paying close attention can reach the solution before the detective states it. Nothing decisive is kept back, and nothing that matters arrives after the reader could have used it.
An old set of rules
This is not a new idea. Almost a hundred years ago the writers of detective stories more or less agreed on a set of rules, some of them written down half in jest, about what an author was and was not allowed to do. The detective could not turn out to be an unmentioned twin produced in the last chapter. The murderer could not use a poison unknown to science that no reader could have anticipated. There could be no hidden passages nobody had spoken of, no answer that leaned on a fact the reader was never shown. Stripped of the jokes, the rules agree that the author must not cheat and that the reader gets a real chance at the answer.
What the lab taught me
I came to fair play from the other direction, through twenty years of failure analysis, and it mapped onto my old work almost exactly. A finding that cannot be shown in the record is not a finding, and in a laboratory that rule outranks being clever. An analyst can be sure a shaft failed from misalignment, and be right, and it still does not count until there is a measurement or a photograph that shows it. Short of that, it is an opinion and not a result, and it does not go in the report as fact. Fair play asks a novelist for the same discipline. If the detective knows something, the reader has to be able to point to where it was shown. Anything else is me asking the reader to take my word for it, and in this kind of book my word is not evidence.
The cost
Fair play forbids the easiest move in the book, the withheld fact. There is always a moment, writing a mystery, when the clean thing would be to keep one fact back. Let the detective notice something the reader was never shown, so the answer lands as a surprise instead of a confirmation. It is tempting every time, because it is so much less work than putting the fact on the page early, in full view, and trusting the reader to look straight at it and not see it. That is the hard part of the job. It means the thing that solves the case has to be there in plain sight for fifty pages, doing nothing anyone notices, until the moment it matters.
Looking at the wrong thing
Misdirection, done fairly, is an arrangement of attention rather than a trick played on the reader. I do not hide the important thing. I put something more interesting next to it. The reader looks at the more interesting thing, because that is what people do, and the important thing stays in view the entire time, ignored, because it looks ordinary and ordinary things do not earn a second glance. When the reader finally turns it over, they find it had been exactly what it appeared to be, and exactly what it was for, since the first page. They simply had a better offer for their attention every time they went past it.
The novel I have coming is built on this rule from the floor up. My detective never knows a thing the reader has not already been shown. That is the promise, and it is why I write this kind of book and not another. Everything is on the page. A sharp reader can get to the end before I take them there, and on the day one of them does, I will count it a job well done.