"My own process of reasoning was not really original," said Miss Marple. "It's all in Mark Twain. The boy who found the horse. I just imagined where I would go if I were a horse and I went there and there was the horse."
"You imagined what you would do if you were a cruel and cold-blooded murderer?" said Craddock, looking thoughtfully at Miss Marple's pink and white elderly fragility. "Really, your mind--"
"Like a sink, my nephew Raymond used to say," Miss Marple agreed, nodding her head briskly. "But as I always told him, sinks are necessary domestic equipment and actually very hygienic."
Agatha Christie, What Mrs. McGillicuddy Saw! Pocket Books (New York: 1958) p. 73. This is often how I work -- I mean, not with a mind like a sink, but by imagining what I would think if, say, I were Berkeley, and then going to see if that's what Berkeley actually thinks. Right or wrong it is always illuminating.
But the allusion to Mark Twain is not ringing any bells.