The gist of the argument was essentially this: murder, one of the worst of all crimes, was only perfect if there were no witnesses, no known motives could be proved, no incriminating evidence was either left at the scene or found on the perpetrator, no connection at all existed between the killer and the victim. Preferably, ideally, there would be no corpse lying around after the event, so that a forensic examination by a doctor or a pathologist was not possible. If any of those conditions was broken, then it was not a perfect murder.

On the other hand, if all those conditions were exactly in place then, ironically, it became a murder not worth writing about in a thriller. Thus the essential contradiction that always frustrated crime writers.

In the real world some people were undoubtedly murdered from time to time in inexplicable circumstances. Killings in this sense would include murders, but were not in themselves mysteries. A body might be found on the side of the road, the victim of a hit-and-run incident, perhaps a deliberate one (murder) or an accident (careless or dangerous driving). The car would bear revealing damage, but if it took several days to be traced, then there was time for the damage to be repaired or the car disposed of. In another example, someone else might be stabbed to death in a fight outside a bar or a nightclub, after which all the witnesses and the perpetrator fled. Or there could be a totally innocent victim shot at random by an armed sociopath who had never killed before, went into hiding and who never killed again.

A murder was often unsolvable, but that did not make it perfect.

Nor did it make it thrilling or mysterious. Such murders were not the stuff of the kind of novels I and all the other crime writers liked to write. Who can solve the unsolvable? Who may be tempted by perfection? Novelists often were. But then novelists sometimes cheated.

Spoder said: ‘Jeksid prepared it as a perfect murder, sir. And I believe it would be unsolved to this day if he had not written this account of it.’

‘Hari Harsent’s wife Frejah knew it must be him. She told me she had him drummed out of the force because of it. That alone removes any claim to perfection. And, as you say, the fact that he has written a confession has the same effect.’

‘His wife Frejah, his widow, accused Jeksid only on suspicion. This is proof.’

‘How long ago was it?’ I said.

‘Nine years. I’m not sure when these notes were written.’

‘The other murders we’ve heard about were even older. They are all beyond the reach of justice now.’

‘Not if Jeksid is still alive. He’s somehow involved in them all.’