Bayes' Theorem
A formula for updating the probability of a belief in light of new evidence — and the basis of modern statistics, medical testing, and a sizable Silicon Valley philosophy.
Origin
Named after Thomas Bayes (1701–1761), an English Presbyterian minister whose essay was published posthumously by his friend Richard Price in 1763. The theorem expresses how to combine your 'prior' belief about something with the strength of new evidence to get a 'posterior' belief. Largely ignored for two centuries, it was rehabilitated in the 20th century and is now the workhorse of medical diagnostics, spam filters, AI, and increasingly the way rationalist communities talk about reasoning under uncertainty.
Modern usage
'Bayesian' as an adjective now means 'reasoning by updating probabilities with evidence.' 'Update your priors' is rationalist-speak for 'change your mind based on new information.' Used (and mis-used) in arguments about everything from COVID testing to courtroom verdicts. Nate Silver's FiveThirtyEight, the Less Wrong community, and most modern forecasting culture are downstream.
In the wild
Given those test results, you should update your priors.— rationalist common usage
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