Hume's Billiard Balls
When one billiard ball strikes another and the second moves, have you actually seen causation, or only a sequence of events?
David Hume posed this challenge in 1748 in his Enquiry Concerning Human Understanding. It targets a concept so fundamental that most people have never thought to question it: the idea that causes necessitate their effects. Hume argued that necessity is something we project onto the world, not something we observe in it.
Hume, D. (1748). An Enquiry Concerning Human Understanding. Section IV. Oxford University Press.
What you actually see
Watch a billiard ball roll across a table and strike a second ball. Here is what you observe: ball A in motion, contact between A and B, ball B in motion. The sequence is clear.
What you do not observe is anything called causation. You see one event followed by another. The idea that A made B move, that there was some necessary connection compelling the second event to follow from the first, is not visible. You added it. Hume's claim is that this addition has no empirical basis.
The regularity theory
If we cannot observe necessity, what is causation actually? Hume's answer is the regularity theory: causation just is constant conjunction. To say A causes B is to say that events of type A are regularly followed by events of type B. Nothing more. No hidden force, no necessary connection in the objects themselves.
The feeling of necessity comes from habit. After watching balls collide hundreds of times, the mind forms a strong expectation. That expectation, projected onto the world, feels like the perception of a real force. But it is a psychological fact about us, not a metaphysical fact about nature.
Why this matters beyond billiard balls
If Hume is right that there is no observable necessity in causation, induction loses its rational foundation. We believe the sun will rise tomorrow because it has risen every day before. But "it has always happened" is not a logical guarantee that it will happen again. The regularity is there, but no necessity ties future to past.
This is Hume's problem of induction, and it has never been fully solved. Kant called it the problem that woke him from his "dogmatic slumber." Karl Popper built his entire philosophy of science around it. The billiard balls sit at the center of one of the most significant unsolved problems in philosophy of science: why should any pattern in the past tell us anything about the future?
Discussion questions
- When you feel certain that pressing a light switch caused the light to come on, what exactly are you certain of?
- If you only ever saw things following each other in sequence, would you ever believe one caused the other?
- Is Hume's doubt about causation liberating or destabilizing to you?
Take it to the dinner table.
Get 3 thought experiments for memorable conversations, designed for dinner, with friends, at events, or anywhere small talk has gone on too long.