A scientist finishes a lecture on the cosmos and an old woman at the back insists the world sits on a giant tortoise. Asked what the tortoise stands on, she replies that it is turtles all the way down. This episode traces that cheeky punchline back to early interpretations of Hindu cosmology, a Jesuit priest named Emmanuel de Vega writing from India around 1599 about seven elephants on a tortoise, and the intellectual dead end his informant hit when asked what held up the foundation. It follows the metaphor through John Locke's attack on the concept of substance and Henry David Thoreau's critique of politicians who lean on one another over nothing.

The hosts then explain how the infinite version emerged, from an 1830s New York Mirror story about rocks all the way down, to William James using it in 1882, to linguist John R. Ross cementing th ...