Source: http://skeptico.blogs.com/skeptico/2008/01/extraordinary-c.html

Extraordinary Claims Require Extraordinary Evidence

So said Carl Sagan. But why? And what is an extraordinary claim?

Strictly speaking, all claims require exactly the same amount of evidence, it’s just that most "ordinary" claims are already backed by extraordinary evidence that you don’t think about. When we say “extraordinary claims”, what we actually mean are claims that do not already have evidence supporting them, or sometimes claims that have extraordinary evidence against them. Extraordinary claims require extraordinary evidence because they usually contradict claims that are backed by extraordinary evidence. The evidence for the extraordinary claim must support the new claim as well as explain why the old claims that are now being abandoned, previously appeared to be correct. The extraordinary evidence must account for the abandoned claim, while also explaining the new one.

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Alien Visitation

Strictly speaking, alien visitation does not contradict other claims that are known to be true. It is theoretically possible that aliens exist and may hold advanced technology that enables them to travel across the galaxy. But the claim is extraordinary in that there is zero evidence alien visitation has actually occurred, despite at least sixty years of looking. In addition, there are rational explanations for many claims of alien visitation. There is no hard evidence of alien visitation, such as a crashed spacecraft with technology far advanced of our own.

David F Mayer