"An extraordinary claim requires extraordinary proof."- Marcello Truzzi
Extraordinary claims require extraordinary evidence because if something is in itself improbable we need compelling evidence before we accept that no one imagined it.
Alice and Bob
Alice and Bob are two friends talking after school. Alice tells Bob that she watched a movie the previous evening. Bob believes her easily, because he knows that movies exist, that Alice exists, and that Alice is capable and fond of watching movies. If he doubts her, he might ask for a ticket stub or a confirmation from one of her friends. If, however, Alice tells Bob that she flew on a Pink Unicorn to a fairy kingdom where she participated in an ambrosia-eating contest, and she produces a professionally-printed contest certificate and a friend who would testify to the events described, Bob would still not be inclined to believe her without strong evidence for the existence of flying unicorns, fairies and ambrosia-eating contests.
Carl Sagan Laplace and Hume
Carl Sagan popularized this as "Extraordinary claims require extraordinary evidence". However, this may have been based on a quote by Laplace which goes, "The weight of evidence for an extraordinary claim must be proportioned to its strangeness." This, in turn, may have been based on the statement "A wise man, therefore, proportions his belief to the evidence" by David Hume.
- Encyclopaedia Galactica | series= Cosmos | credits= Carl Sagan (writer/host) | network= PBS | airdate= December 14, 1980 | number= 12 | minutes= 01:24
- book | last = Hume| first = David| authorlink = David Hume| title = An Enquiry concerning Human Understanding, chap. 10.4 | year=1748 | url=http://www.davidhume.org/texts/?text=ehu#10