Tuesday, October 11, 2016

Absence of Evidence Is Evidence of Absence

Carl Sagan claimed that "Absence of evidence is not evidence of absence". From what I gather, it was in reply to people who claimed that lacking evidence for alien life meant this didn't exist. Much as I respect Sagan, however, his claim is wrong. Lack of evidence is evidence that something is not there, it is just not proof. This may seem to be a semantic issue regarding what evidence and proof mean. Nonetheless, it seems to me that being clear in our terminology is very important. Much as in the similar slogan "Extraordinary claims require extraordinary evidence" which I examined in my last post (one Sagan also used) a confusion in our terms renders these hopelessly subjective and thus useless.

Evidence means something that tends to prove or disprove, or that grounds a belief. Proof on the other hand is evidence sufficient to establish that something is true, or produce such a belief. Thus, lack of evidence for aliens doesn't prove they don't exist. It does however tend in the direction. Of course a believer in aliens may certainly have some evidence for them as well. We must weigh any evidence for or against this. Often we do not have proof, or at least none which is strong. Therefore, though absence of evidence is not proof of absence, it is evidence of absence. A difference between these is real, and as is said, makes all the difference. They should not be confused in this way. 

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