Reading an old mystery (Erle Stanley Gardner, The Case of the Lame Canary), ran across this passage:
Mason said, “That’s the puzzling part of it, Della. He should have been the murderer instead of the corpse.”
“This,” Della Street pointed out, “isn’t getting us any nearer China.”
“I think it is,” Mason said thoughtfully. “It sounds foolish, and yet I think it’s getting me some place. It’s paradoxical. The man who was murdered isn’t the man who was murdered, but the man who committed the murder. Now, if we can follow that contradictory premise through to a logical conclusion, Della, we’re certainly going to be one jump ahead of the police, because that’s a starting point of deductive reasoning which would never suggest itself to them.”
"No, " she admitted a smile, “you win on that.”
…A series of knocks sounded on the door which led to the corridor. Mason said, “That’s Paul Drake. Let him in, Della, and see what he wants.”
Della Street crossed the room and opened the door.
"Hello, folks, " Drake said. “What’re you doing?”
“We’re engaging in a new form of logic,” Della answered a grin. “It’s swell. It solves murders and everything.”
"Gimme, " Drake said, entering the room.
“Well, it goes like this,” Della said. "Because you’ve come in the room, you must have been the person going out of the room. Therefore, having gone out of the room while you were coming into the room, someone who saw you in the corridor coming into the room, would have you were going out of the room, and - "
“Oh, I see,” Drake said, “like a puppy chasing his tail, huh?”
“Exactly,” Della agreed, "only the puppy catches his tail. Then, having swallowed himself, he becomes, so to speak, completely self-contained. "
