The problem of induction

Main points

Induction is the process of reasoning from one set of observations to conclusions about things that haven’t been observed. We use induction when we make predictions about the future or offer explanations about the unobserved events that led up to the present. Hume’s problem of induction seems to show that we have no reason to draw these inferences.

The need for a connecting proposition

Hume observes that we move from observations to conclusions about things that are not observed. But why are observations about how things worked in the past relevant to how they will behave in the future? Hume argues that we take for granted what he variously calls a “connecting proposition” or “medium” that I have called the uniformity principle.

1. Observations: “I have found that such an object has always been attended with such an effect” (p. 22). For example, ‘I have found that the the cue ball’s striking the 8 ball has always been attended with the 8 ball moving away from the cue ball.’
2. Uniformity Principle: “the future will resemble the past, and … similar powers will be conjoined with similar sensible qualities” (p. 24) Or, “the course of nature will continue uniformly the same.” (p. 130)
3. Conclusions: “I foresee that other objects, which are in appearance, similar, will be attended with similar effects.” (p. 22) For example, when I am trying to make a shot in pool, I think, ‘I forsee that making the cue ball strike the 8 ball will be attended with the effect of moving the 8 ball away from the cue ball’ and plan my shot accordingly.

You have plenty of reasons to believe what you have observed: you saw it with your own eyes. So far so good. But you will only have reasons to believe the conclusion if you have reasons to believe the uniformity principle. That’s where the trouble lies.

Why believe the uniformity principle?

We cannot tell that it is true on the basis of the relation among ideas. This is because it is conceivable that nature might not be uniform. For instance, you could imagine the the laws of gravitation change so that we would be like astronauts in space even down here on earth. There is nothing wrong with the combination of ideas you would have in imagining that. By contrast, you cannot imagine that 3 ≠ 3.

But, you say, it isn’t really possible. After all, we have good reason to believe that the laws of nature never change. ‘What is that good reason?’, Hume might ask. If you say ‘because the laws of nature have never changed in the past,’ you’re in trouble.

What you would be saying would be:

1. We have observed that the laws of nature have never changed in the past.
2. So we know the laws of nature will not change in the future.

And Hume will ask: ‘Why are your past observations relevant to how things will be in the future?’

You had better not answer: ‘Because the course of nature will continue to be the same.’ That’s what you’re trying to prove. But what else can you say?

That’s the problem of induction. In order for observations of the past to give you any reason to believe something about the future, you have to have reason to believe in the uniformity principle. But you do not have any reason to believe in the uniformity principle.

Key concepts

1. Examples of induction: observations of the past leading to conclusions about the future or observations of the present leading to conclusions about the past.
2. Why Hume thinks we need the uniformity principle.
3. Why he thinks we do not have any reason to believe the uniformity principle.