X0 is turtles, as recorded in Stephen Hawking’s 1988 A Brief History of Time:
A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: “What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise.” The scientist gave a superior smile before replying, “What is the tortoise standing on?” “You’re very clever, young man, very clever”, said the old lady. “But it’s turtles all the way down!
X must be something that can be pluralized: actors, cans, features, caches, big bangs, cronies, and fraudsters all occur in the variable position. It often seems that if a person suggests that it’s “caches all the way down“, he means that many things in a system may be called something else but ultimately can be modeled as caches. Whether we think of gravity and centrifugal force rather than the balancing of a giant turtle as what holds us on the earth, it doesn’t make much difference in most of our day-to-day actions. I’ve also often heard “it’s X all the way down” as the answer to “but then what? but then what?” type questions–at some point the last thing you understand in a sequence of events is what goes all the way down.