Sagan standard

Carl Sagan popularized the maxim in the 1980 PBS series Cosmos and his 1995 book The Demon-Haunted World, but the underlying idea is older. Pierre-Simon Laplace stated essentially the same principle in 1814 (“the weight of evidence for an extraordinary claim must be proportioned to its strangeness”), and David Hume's argument against miracles in the 1748 Enquiry runs on the same logic: a wise person proportions belief to evidence.

In Bayesian terms, the standard is just a consequence of Bayes' theorem. The posterior probability of a claim depends on its prior probability multiplied by how well the evidence fits. If the prior is very low (the claim is extraordinary), the evidence has to be correspondingly strong to lift the posterior to a credible level. Eyewitness testimony that would be sufficient for an ordinary claim ("I had cereal for breakfast") is insufficient for an extraordinary one ("I had breakfast with a resurrected first-century Jewish carpenter").

The standard is heavily used in skeptical investigations of paranormal claims, alternative medicine, and cryptozoology. It is also a recurring move in arguments about miracles and the historicity of religious events. Defenders of religious claims often respond by either contesting the prior (the claim is not as extraordinary as the skeptic supposes, given a theistic background) or contesting the evidence (the case for the resurrection, for instance, is stronger than skeptics admit). The Sagan standard does not settle these debates; it just clarifies what "strong evidence" needs to mean.

A common abuse of the standard is to use it as a thought-terminating cliche — declaring a claim "extraordinary" without saying why, and demanding evidence so high no testimony could meet it. Used carefully, the principle is just probability theory; used carelessly, it becomes a slogan that begs the question. The discipline is to make the prior, the alternatives, and the evidence explicit, and to follow them where they go.