Dialectician 1.   g31 Extraordinary claims require extraordinary evidence.

Archae.  g32 [vs. g31] The Extraordinary Claims argument is so vague and so general, that it could probably be used convincingly, but not always legitimately, to dispose offhandedly of any and all evidence for any extraordinary claim whatever. g33 It might be said against the most common use of this principle that any good evidence for extraordinary claims is extraordinary evidence, but let us examine the principle more closely.

g34 The principle (first articulated by Carl Sagan) is this: Extraordinary claims require extraordinary evidence. g35 But what, precisely, does extraordinary mean here? In the phrase "extraordinary claims," "extraordinary" cannot simply mean "astounding" or "fascinating," or even "unexpected"; rather, it can only mean "improbable", and in the phrase "extraordinary evidence," the word can only mean "unusually strong." Thus the principle actually amounts to this:


g36 Improbable claims require unusually strong evidence.


g37 The principle seems clearly to be a true principle. g38 But, thus clarified, we can see that it does not apply to any claim unless there is something improbable about the claim.  


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