Bayes’s Theorem: the mathematical formula that ‘explains the world’

Here’s a profound question about beards: is the number of acrobats with beards the same as the number of bearded people who are acrobats? Go with your gut instinct. It’s not a trick question. If you answer ‘yes’, then you’ve understood the central idea behind Bayes’s theorem. If you’re one of those people who likes to titter about how bad you are at mathematics, stop it. Retake your GCSE, learn how to pin this obviousness down in symbols, and you can produce artificial intelligence, forecast stock market collapses and understand this:

P(A|B) = (P(B|A)∙P(A))/(P(B))

This is Bayes’s equation, the formula which, as Tom Chivers insists in this remarkable, bold book, ‘explains the world’.

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Rich mathematics begins by spotting an oddity in the obvious.