Origins of the saying, "Extraordinary claims require extraordinary evidence."

The saying, "Extraordinary claims require extraordinary evidence," which Carl Sagan popularized, has a heritage. Sagan's phrasing was inspired by Marcello Truzzi, founding co-chairman of the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP):

"In science, the burden of proof falls upon the claimant; and the more extraordinary a claim, the heavier is the burden of proof demanded. The true skeptic takes an agnostic position, one that says the claim is not proved rather than disproved. He asserts that the claimant has not borne the burden of proof and that science must continue to build its cognitive map of reality without incorporating the extraordinary claim as a new "fact." Since the true skeptic does not assert a claim, he has no burden to prove anything. He just goes on using the established theories of "conventional science" as usual. But if a critic asserts that there is evidence for disproof, that he has a negative hypothesis—saying, for instance, that a seeming psi result was actually due to an artifact—he is making a claim and therefore also has to bear a burden of proof."

Marcello Truzzi, On Pseudo-Skepticism, Zetetic Scholar, 12/13, pp3-4, 1987