mystery. I do not say that deductions are not helpful, but they are not all. A great writer once compared the science of detection to a game of cards, and the comparison is very accurate. A good player can judge, with reasonable certainty, the cards in the hands of each of his opponents. But he can never be absolutely certain—especially when he is unacquainted with his opponents' methods of play.
"Detection can never be reduced to a mathematical certainty until you level human nature, so that every person in the same set of circumstances will act in exactly the same way. Like doctors, we have to diagnose from circumstances—and even the greatest doctors are wrong at times. Specialist knowledge has often to be called in.
"When this case commenced, specialist knowledge had to be enlisted to fix our facts—and the one general difficulty which arose as always, was that we did not know which facts might prove important. As an instance, I may say that the finger-prints on the dagger were wholly misleading, and might have brought about a miscarriage of justice.
"It was necessary that we should collect every fact we could about the murder, whether great or small. That was one phase for the investigation where organisation was necessary. A man working alone would have taken months, perhaps years, in this preliminary work. Then luck favoured us. Our records—collected, of course, by organisation—contained a portrait of a man strikingly like you"—he nodded to Grell—"and a comparison of finger-prints told us that the dead man was not you, but Harry Goldenburg.
"Previously, the time of the murder had been fixed by Professor Harding as between ten and twelve. It was our business to find out who had been with Harry Goldenburg at that time. Among those persons was the guilty one."
"I can't see how that helped you at all," said Grell, his brows bent.
"In this way, and as a negative test. The alibi is a commonplace of the criminal courts. Every person on whom clues might ultimately rest would be eliminated from the investigation if it could be proved beyond doubt that they were elsewhere at the time. You must remember, that we had not only to find the murderer, but to produce evidence that would satisfy a jury that we were right. But we worked, first of all, from such main facts as we had. You were missing. Ivan was missing. A mysterious veiled woman was missing. There was the pearl necklace that you had bought as a wedding present for Lady Eileen. There was the strange dagger used in the murder. There was the miniature of Lola on the dead man. These were the chief heads. There were scores of minor things to be dealt with.
"The matter was complicated, too, by the dead man's clothes. In the pockets, there were your personal belongings. A natural, but erroneous assumption was that they were your clothes. There is not much scope for individuality in evening dress. I confess I was misled and puzzled at first, but a little thought afforded the explanation, and, in fact, it would have been cleared up automatically in any event by the examination of the garments.