After delivering a lecture on the solar system, philosopher-psychologist William James was approached by an elderly lady who claimed she had a theory superior to the one described by him.
"We don't live on a ball rotating around the sun," she said. "We live on a crust of earth on the back of a giant turtle."
Not wishing to demolish this absurd argument with the massive scientific evidence at his command, James decided to dissuade his opponent gently.
"If your theory is correct madam, what does this turtle stand on?"
"You're a very clever man, Mr. James, and that's a good question, but I can answer that. The first turtle stands on the back of a second, far larger, turtle."
"But what does this second turtle stand on?" James asked patiently.
The old lady crowed triumphantly, "It's no use, Mr. James -- it's turtles all the way down!"
With permission from "Natural History," June-July, 1974. (c) the American Museum of Natural History, 1974.m