Imagine you’re looking for your keys and you think you might have left them on the bookshelf. But when you look, you see nothing but books. A natural conclusion to draw is that the keys are not there.

Now imagine you’re an early 20th century astrophysicist seeking to test the hypothesis that there is a planet (Vulcan) causing perturbations in Mercury’s orbit. You keep looking but find nothing. You conclude that Vulcan does not exist.

Both arguments seem straightforward, and yet in both cases you are relying on an assumption that an absence of evidence can be a good reason for inferring that what you are looking for is just not there.

Read more: Sorry Mr Spock: science and emotion are not only compatible, they're inseparable

In other words, an absence of evidence is evidence of absence.

But it’s the opposite assumption — that an absence of evidence is not evidence of absence — that has come to have the status of a received truth.

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