Corlett Novis
Jul 4, 2018 · 4 min read

Consider the arrangement of a simple game of snooker. Nothing fancy, just two smooth billiard balls, one white and one red, arranged on a cloth covered table. Suppose we strike the white ball with a cue, sending it with some speed towards the red ball at the end of the table. It strikes.

What happens next?

This thought experiment may appear basic, you don’t even need to know much about physics to understand the simple dynamics at work. The white ball strikes the red one and stops. The force of the strike sends the red ball forward. Simple. We may be tempted to stop there, but let’s consider another scenario. The white ball collides with the red ball and stops. But rather than moving, both balls remain stationary, resting unperturbed on top of the table. In theory, both of these scenarios seem plausible, so what makes one seem more likely than the other? More importantly, is there any real way of establishing which will occur?


This experiment was designed to explore what we know about causality, the relationship between causes and their effects. It was invented almost 300 years ago by the Scottish philosopher David Hume. Hume pointed out that something very peculiar was wrong with the way we viewed the world around us, a problem which has endured both for scientific and philosophical thinkers alike: “The Problem of Induction”.

The reason we believe that when the white ball strikes the red one it will send the latter forward is because, in the past, this is what has happened. Every time we’ve seen billiards, pool or snooker played, a collision between a moving ball and a stationary one has caused the stationary ball to move forward. As with billiards, so too with science. We assume that the movement of planets, the course of chemical reactions and even biological processes will continue to happen in the future the way they have in the past. For Hume, however, this isn’t enough. Just because we’ve observed uniform, predictable and periodic events in the past this doesn’t mean they will continue that way. Lets consider this argument:

  • In the past, every time the white ball hit the red one, the red ball moved.
  • Therefore, when the white ball hits the red one, the red one will move.

But this is where things get a little complicated. Hume points out that there is actually a hidden premise in this argument. The hidden premise is called the “principle of uniformity” which states that events in the future will resemble events we’ve seen in the past. In this way:

  • In the past, every time the white ball hit the red one, the red ball moved.
  • Principle of uniformity: events in the future will resemble events of the past
  • Therefore, when the white ball hits the red one, the red one will move.

So far so good. But what justification do we have for the so called “principle of uniformity”? This is where the difficulty arrises, because according to Hume the principle is actually self justifying. In other words, the only reason we believe in the principle of uniformity is because of the principle of uniformity itself!

  • In the past, the principle of uniformity has been true.
  • Principle of uniformity: events in the future will resemble events of the past
  • Therefore, the principle of uniformity will continue to be true.

What we are left with is a cyclical argument in which the principle self-justifies and is considered logically invalid.

So what exactly does that mean? It means things look pretty bleak. As it stands, this argument demonstrates that my prediction that the white ball will move the red ball is just as reasonable as saying the white ball will suddenly turn into an egg and hatch once it hits the red one. An astronomer’s prediction that a comet will pass by is just as likely as a prediction that the moon will suddenly vanish into a cloud of smoke. We can no longer logically guarantee that causes will result in particular effects. Anything can happen. Nothing is predictable.


This argument, formulated in 18th century Scotland, is an enduring issue to this day. It attacks the very logical basis of science: induction. Induction allows us to draw generalised rules from specific empirical observations but, as demonstrated above, there is no logical basis for it. That’s why the thinker C. D. Broad once noted:

“induction is the glory of science and the scandal of philosophy.”

But how do we resolve this scandal? This has been the task of scientists, theologians and philosophers to this day, but Hume provides us at least some resolve. Perhaps unexpectedly, Hume puts an emphasis on custom rather than reason. Given our present, and indeed historical, cultural obsession with intelligence and reason it is certainly refreshing to see Hume resolve this problem with reference to one of our less popular faculties. David Hume points out that we use induction not out of reason, but out of custom, and without custom “we would be entirely ignorant of every matter of fact beyond what is immediately present to the memory and senses.”

Thanks for making it to the end. If you found this interesting, I’d encourage you to do some of your own research on the “problem of induction” and decide for yourself whether induction can be justified.

Meaning of the Method

Stories investigating the sciences

Corlett Novis

Written by

Editor at Pi Media (London) interested in Science and Technology and how they interact in wider society and culture.

Meaning of the Method

Stories investigating the sciences

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