On Probability

Big idea

Summary

Lewis engages the philosopher David Hume's celebrated argument against believing any report of a miracle: since our uniform experience testifies that the laws of nature hold without exception, while our experience of human testimony shows it to be frequently mistaken or dishonest, the balance of probability will always favor disbelieving the miracle report, however reliable the particular witnesses seem. Lewis grants the argument its full force as far as it goes, but presses on where its 'uniform experience' actually comes from.

His central objection is that Hume's argument quietly assumes what it needs to prove. To know that experience is uniformly against miracles, we would already need to know that no reported miracle has ever actually occurred; but that is precisely the question in dispute. If even one miracle has ever happened, then experience is not, in fact, uniform, and Hume's whole argument collapses into circularity — it works only by silently building its own conclusion into its starting premise.

Lewis further argues that probability, properly understood, is not simply a matter of counting how often something has happened before; it also depends on our total picture of what kind of universe we live in. Once the earlier chapters have established that Nature is not the whole of reality, and that miracles clustering around the Incarnation would be exactly the kind of thing to expect if Christianity's larger story is true, the antecedent probability of the Gospel miracles rises considerably — not because ordinary events have become less regular, but because we now have independent reason to expect this particular exception at this particular point in the story.

The chapter's conclusion is not that miracle reports should therefore be accepted uncritically; ordinary historical scrutiny of specific evidence still matters enormously, and Lewis will apply it directly to the Resurrection in a later chapter. The point is narrower but decisive: no one can rule out miracles in advance on grounds of probability alone, because that supposed 'uniform experience' against them is available only to someone who has already assumed the conclusion he claims to be proving.

Key points

• Hume's classic argument holds that uniform experience against miracles will always outweigh even strong testimony for one, but this 'uniform experience' can only be known if we already know no miracle has ever occurred — which is the very question in dispute.
• The argument is therefore circular: it assumes its conclusion (miracles never happen) in order to establish its premise (experience is uniformly against them).
• Probability is not simply a tally of past frequency; it also depends on our total picture of the universe, so establishing that Nature is not the whole of reality already raises the antecedent probability of a genuine miracle.
• Lewis is not asking readers to accept miracle reports uncritically — ordinary historical evidence still has to be weighed — only denying that probability alone can rule miracles out before any evidence is examined.

Illustrations & analogies

• Hume's image of weighing testimony on one side of a scale against 'uniform experience' on the other is examined closely, and shown to load the scale in advance by assuming, rather than establishing, that the far side always wins.

For reflection and discussion

1. Had you encountered Hume's argument against miracles before, in this or another form? Does Lewis's charge of circularity seem fair to you?
2. Lewis says our overall picture of the universe affects how probable we judge a miracle to be, even before we look at specific evidence. Do you think that is a legitimate way to reason, or does it risk deciding the answer in advance either way?