In the golden age of mystery writing there was the idea that a murder mystery was a game between the author and the reader. The author would give all of the clues necessary and would win if the reader couldn’t guess the solution before the detective reveals it; the reader would win if he guessed first.
The idea was all over the place at the time. For example, you can see aspects of this in Fr. Ronald Knox’s 10 Commandments of Detective Fiction. Most of the rules ensure that the necessary evidence is actually given to the reader.
Fair play is a great idea, and makes for good books. The only problem is that it—mostly—doesn’t actually work for its intended purpose.
The problem with “fair play” in murder mysteries, as a game, is that there are usually too many degrees of freedom. Ambiguity can be resolved in too many ways. For example, it is possible that the murderer managed to pretend to come into the room he was already in, but then it was also possible someone in the séance circle holding hands with the victim could have let go and stabbed the victim before he could cry out. Neither is probable and neither is impossible. Which is more likely? The reader is not really solving the problem, but instead reading the mind of the author.
This sort of ambiguity comes up a lot because ambiguity is the heart of mystery. If there’s only one interpretation, there’s nothing to detect, or at least there’s no value in the detective being brilliant. More than that, there is always ambiguity so long as there is a next page—it is always possible that the next page will contain new evidence which changes the meaning of evidence which showed up before.
On the flip side, if sufficient evidence to obtain a criminal conviction is given prior to the suspects gathering in the accusing parlor, the game will be too easy. If there is insufficient evidence for a conviction, the solution will be unsatisfying. It seems like there should be a happy medium, but there isn’t. The most common attempt is for the detective to withhold from the reader some conclusive peace of evidence and only present it when the formal accusation is made. This does not solve the problem, though, because it is almost always the case that a different piece of conclusive evidence could be given for at least one other suspect. In short, if there is any point in guessing, there will always be more than one reasonable guess.
Fair play doesn’t really work for its intended purpose, but it does make for good book.
There are two main reasons for this. The first is that fair play unites the reader with the detective. The second is that fair play forces the writer to make the mystery mysterious because it is clever.
When the author hides clues from the reader, it means that the reader doesn’t know why the detective does what he does. The detective, himself, becomes a mystery. In this case the reader is united, not to the detective, but to the Watson. (To some degree this explains the original Watson, who was, in theory, the author of the stories and hence they were from his perspective.) On the plus side, this multiplies the mysteries. The downside is that it makes the detective an unsympathetic character, in the strict sense that the reader cannot sympathize with him.
Interestingly, Sherlock Holmes was originally meant to be an unsympathetic character in the other sense—aloof, unapproachable, eccentric in the extreme. It did not last; Holmes was humanized, over time. Part of this was that there were often sub-mysteries which Holmes would explain, allowing us to get close to the detective.
The other aspect of fair play—that it forces the writer to write good mysteries—is also important to a good novel. A mystery in which the mystery is maintained only by withholding clues is, generally, a very simple mystery. If there is nothing much for the detective to think about, the solution will not be very satisfying when it is finally offered.
Another effect fair play has on plot construction is that it forces the author to be careful with the rate at which clues are given, and the circumstances which produced them. If the author has to reveal what the clue is when the reader has time to think about it, it being unreasonable for the culprit to have left the clue will stand out more.
For these reasons, and probably for others, too, fair play is as important now as it ever was. I think we merely lack the societal explanation which was never really adequate, even in its own day.