Why do “extraordinary claims require extraordinary evidence”?
This is a common expression among scientists who are challenged to disprove some unlikely theory that overthrows accepted theories. As theories continue to have repeated success at predicting future observations, they develop a higher track record of reliability. If some new theory comes along that tries to overthrow the existing theory, it must have an even higher level of evidence to overthrow previous successes of the current theory. The more accepted and better tested a theory is, the higher the threshold will be for the evidence required to refute it. For more information, see The Scientific Method FAQ published by the sci.skeptic newsgroup.
