"It's all very well sitting round with your large disguised policemen outside Mrs. Forrest's flat," he said irritably, over bacon and eggs on Monday morning, "but you do realize, don't you, that we've still got no proof of murder. Not in one single case."
"That's so," replied Parker, placidly.
"Well, doesn't it make your blood boil?" said Wimsey.
"Hardly," said Parker. "This kind of thing happens too often. If my blood boiled every time there was a delay in getting evidence, I should be in a perpetual fever. Why worry? It may be that perfect crime you're so fond of talking about - the one that leaves no trace. You ought to be charmed with it."
(from Unnatural Death, by Dorothy L. Sayers)