“I shall take up your contention,” said Quinny without pause for breath, “first, because you have opened up one of my pet topics, and, second, because it gives me a chance to talk.”  He gave a sidelong glance at Steingall and winked at De Gollyer.  “What is the peculiar fascination that the detective problem exercises over the human mind?  You will say curiosity.  Yes and no.  Admit at once that the whole art of a detective story consists in the statement of the problem.  Any one can do it.  I can do it.  Steingall even can do it.  The solution doesn’t count.  It is usually banal; it should be prohibited.  What interests us is, can we guess it?  Just as an able-minded man will sit down for hours and fiddle over the puzzle column in a Sunday balderdash.  Same idea.  There you have it, the problem—­the detective story.  Now why the fascination?  I’ll tell you.  It appeals to our curiosity, yes—­but deeper to a sort of intellectual vanity.  Here are six matches, arrange them to make four squares; five men present, a theft takes place—­who’s the thief?  Who will guess it first?  Whose brain will show its superior cleverness—­see?  That’s all—­that’s all there is to it.”

“Out of all of which,” said De Gollyer, “the interesting thing is that Rankin has supplied the reason why the supply of detective fiction is inexhaustible.  It does all come down to the simplest terms.  Seven possibilities, one answer.  It is a formula, ludicrously simple, mechanical, and yet we will always pursue it to the end.  The marvel is that writers should seek for any other formula when here is one so safe, that can never fail.  By George, I could start up a factory on it.”

“The reason is,” said Rankin, “that the situation does constantly occur.  It’s a situation that any of us might get into any time.  As a matter of fact, now, I personally know two such occasions when I was of the party; and devilish uncomfortable it was too.”

“What happened?” said Steingall.

“Why, there is no story to it particularly.  Once a mistake had been made and the other time the real thief was detected by accident a year later.  In both cases only one or two of us knew what had happened.”

De Gollyer had a similar incident to recall.  Steingall, after reflection, related another that had happened to a friend.

“Of course, of course, my dear gentlemen,” said Quinny impatiently, for he had been silent too long, “you are glorifying commonplaces.  Every crime, I tell you, expresses itself in the terms of the picture puzzle that you feed to your six-year-old.  It’s only the variation that is interesting.  Now quite the most remarkable turn of the complexities that can be developed is, of course, the well-known instance of the visitor at a club and the rare coin.  Of course every one knows that?  What?”

Rankin smiled in a bored, superior way, but the others protested their ignorance.

“Why, it’s very well known,” said Quinny lightly.