After a lecture on the solar system, philosopher William James was approached by an elderly lady.
"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."
James asked "If your theory is correct, madam, what does this turtle stand on?"
"The first turtle stands on the back of a second, far larger turtle, of course."
"But what does this second turtle stand on?"
The old lady replied "It's no use, Mr. James -- it's turtles all the way down!"