There is an idea prevalent that a detective story is rather like a big race—a number of starters—likely horses and jockeys.
“You pays your money and you takes your choice!” The favourite is by common consent the opposite of a favourite on the race course. In other words he is likely to be a complete outsider! Spot the least likely person to have committed the crime and in nine times out of ten your task is finished.
Since I do not want my faithful readers to fling away this book in disgust, I prefer to warn them beforehand that this is not that kind of book. There are only four starters and any one of them, given the right circumstances, might have committed the crime. That knocks out forcibly the element of surprise. Nevertheless there should be, I think, an equal interest attached to four persons, each of whom has committed murder and is capable of committing further murders. They are four widely divergent types, the motive that drives each one of them to crime is peculiar to that person, and each one would employ a different method. The deduction must, therefore, be entirely psychological, but it is none the less interesting for that, because when all is said and done it is the mind of the murderer that is of supreme interest.
I may say, as an additional argument in favour of this story, that it was one of Hercule Poirot’s favourite cases. His friend, Captain Hastings, however, when Poirot described it to him, considered it very dull! I wonder with which of them my readers will agree.
[Cards on the Table -Agatha Christie] Foreword