Extraordinary claims require extraordinary evidence.

Carl Sagan popularized this maxim in the late Cold War era as part of his public campaign for scientific skepticism and critical thinking. It appears prominently in his 1995 book *The Demon-Haunted World*, written amid widespread public fascination with UFOs, paranormal claims, and conspiracy thinking, and alongside Sagan’s concern that scientific literacy was eroding. The line encapsulates a core norm of scientific reasoning: when a claim would, if true, overturn well-established knowledge (e.g., physics, biology, history), the evidential bar must rise accordingly. Sagan used it to frame how to evaluate sensational assertions without cynicism—by proportioning belief to the quality and weight of evidence.

The quote argues for proportionality between what is being asserted and what must be shown. “Extraordinary” does not mean merely surprising; it means a claim that conflicts with robust, converging bodies of prior evidence or would require revising fundamental explanations. In such cases, ordinary testimony, anecdotes, or weak correlations are insufficient; one needs unusually strong, replicable, and independently verifiable support. The principle is not a refusal to accept novel ideas—science advances through them—but a safeguard against being misled by error, bias, fraud, or wishful thinking. It also implies that skepticism is methodological: disbelief is not automatic, but calibrated to evidential standards.

1) “Extraordinary claims require extraordinary proof.”
2) “Extraordinary claims require extraordinary evidence.” (also commonly cited as “Extraordinary claims demand extraordinary evidence.”)