In 1988 I was given Stephen Hawking’s famous book A Brief History of Time which was the catalyst for my unrequited love of physics. The book opens with a curious anecdote:

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.”

Some version of this anecdote has been around since the 17th century. What is the point of this story? It’s not that the earth is actually flat (even though some people actually believe that, see this IFOD: Flat Earth Society). Rather it deals with the concept of infinite regress.

Infinite Regress according to Dictionary.com is “a sequence of reasoning or justification that can never come to an end.” It’s philosophical problem -can there really be a justification for something where there’s no root cause, just a series of prior causes stretching out to infinity?

Here’s a great example from the website Answers in Reason: If we imagine a soldier waiting for orders from the soldier before to fire at the enemy. The soldier at the front asks the soldier behind if they have permission to fire. That soldier then asks the soldier behind them, then that soldier repeats the same process. Eventually we must come to a soldier that gives permission to fire, otherwise the soldier at the front of line would never be able to fire. There must be a soldier who is the ‘first cause’, the one that gives permission to fire.

This problem of Infinite Regress is often applied to the origin of life and the universe to argue that at some point something must have been the cause of it all. An argument in this vein is Aristotle’s Unmoved Mover Paradox whereby he notes that the whole universe is in motion and to be in motion objects have to be set in motion by something – they don’t start moving on their own. While one moving thing can start another object moving, at some point Aristotle reasoned that there must be a stationary thing – an “unmoved mover” that set everything in motion.  This is interesting, but it is a paradox because it doesn’t answer the question “who created the unmoved mover?” To say the “unmoved mover just is and always has been” doesn’t solve the infinite regress problem. It’s like saying “it’s turtles all the way down.”

I love this concept of turtles all the way down and it has become one of my primary mental models. My take is that we all have our turtles – it’s just a matter of how far down they go until we hit a big levitating turtle — whether it’s “faith” or just admission of our ignorance. This picture hangs on the wall of my office to remind me of this mental model: