You complained, cher ami, that I was "unfair" in my presentation of this case. I withheld from you knowledge that I had myself. That is to say, I refused to tell you the identity of X. That is quite true. I had to do so — though not for the reasons that I advanced. You will see the reason presently.

And now let us examine this matter of X. I showed you the resume of the various cases. I pointed out to you that in each separate case it seemed quite clear that the person accused, or suspected, had actually committed the crimes in question, that there was no alternate solution. And I then proceeded to the second important fact

— that in each case X had been either on the scene or closely involved. You then jumped to a deduction that was, paradoxically, both true and false. You said that X

had committed all the murders.

But, my friend, the circumstances were such that in each case (or very nearly) only the accused person could have done the crime. On the other hand, if so, how account for X? Apart from a person connected with the police force or with, say, a firm of criminal lawyers, it is not reasonable for any man or woman to be involved in five murder cases. It does not, you comprehend, happen! Never, never does it occur that someone says confidentially: "Well, as a matter of fact, I've actually known five murderers"! No, no, mon ami, it is not possible, that. So we get the curious result that we have here a case of catalysis — a reaction between two substances that takes place only in the presence of a third substance, that third substance apparently taking no part in the reaction and remaining unchanged. That is the position. It means that where X was present, crimes took place — but X did not actively take part in these crimes.

An extraordinary, an abnormal situation! And I saw that I had come across at last, at the end of my career, the perfect criminal, the criminal who had invented such a technique that he could never be convicted of crime.

It was amazing. But it was not new. There were parallels. And here comes in the first of the "clues" I left you. The play of Othello. For there, magnificently delineated, we have the original of X. Iago is the perfect murderer. The deaths of Desdemona, of Cassio — indeed of Othello himself — are all Iago's crimes, planned by him, carried out by him. And he remains outside the circle, untouched by suspicion — or could have done so. For your great Shakespeare, my friend, had to deal with the dilemma that his own art had brought about. To unmask Iago, he had to resort to the clumsiest of devices — the handkerchief — a piece of work not at all in keeping with Iago's general technique and a blunder of which one feels certain he would not have been guilty.