“Then you would be unreasonable,” said Gordon. “In the old days, when the answers of sums used always to come out to an integral number—I believe it’s all different now—one was wiser than that. If you could see that the answer was going to involve two-thirds of a policeman, you argued at once that you were on the wrong track; you started again, and suspected your working.”

“But real life,” retorted Reeves, “doesn’t always work out to a simple answer. And if the policeman who’s in charge of a case argues as you’re arguing, he’s only himself to blame for it if he gets trisected by the criminal before he’s finished.”

“At least you must respect the principle of Cui bono?”

“It’s extraordinary,” began Carmichael, “how many people make the old mistake about the meaning of——”

Cui bono is the worst offender of the lot,” said Mordaunt Reeves cheerfully. “Look at those two boys in America who murdered another boy just to find out what it felt like.”

“But that was pathological.”

“And how many crimes aren’t pathological, if it comes to that?”

“I was on Holy Island once for a month,” said Carmichael, “and would you believe it, there was a man there that was sick if he ever caught sight of a dog? Sick, positively.”

“What do you think it really feels like,” asked Marryatt, “to have murdered a man? I mean, murderers in general always seem to lose their heads when the thing is actually done, and give themselves away somehow. But one would have thought, if the thing is planned with proper deliberation, one’s feeling would be that things were working out according to plan, and the next thing was to get clear—above all things, to see plenty of people, and to behave quite naturally in company.”

“Why that?” asked Gordon.