Two wrongs don't make a right, but three lefts do. --- Anon.

After a lecture on the solar system, philosopher William James was approached by a determined elderly lady with a theory. "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 decided to be gentle. "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 crowed triumphantly. "It's no use, Mr. James -- it's turtles all the way down!"

Source: American Museum of Natural History