After a lecture on cosmology and the structure of the solar system, William James was accosted by a little old lady who told him that his view of the earth rotating round the sun was wrong.

“I've got a better theory,” said the little old lady.

“And what is that, madam?” inquired James politely.

“That we live on a crust of earth which is on the back of a giant turtle.”

“If your theory is correct, madam,” he asked, “what does this turtle stand on?”

“You're a very clever man, Mr. James, and that's a very good question,” replied the little old lady, “but I have an answer to it. And it's this: the first turtle stands on the back of a second, far larger, turtle, who stands directly under him.”

“But what does this second turtle stand on?” persisted James patiently.

To this, the little old lady crowed triumphantly, “It's no use, Mr. James, it's turtles all the way down.”