Absence of Evidence (The limitations of science)

by whitecraw, Wednesday, February 27, 2008, 18:48 (6114 days ago) @ George Jelliss

'One or two contributors have cited the unaccountably popular principle that "Absence of evidence is not evidence of absence". - 'With respect I must challenge the veracity of this principle.' - The popular principle points to the logical fallacy known as argumentum ad ignorantiam, in which it is claimed that a statement is true only because it has not yet been shown to be false or is false only because it has not been shown to be true. Two examples of this fallacy are the statements 'You can't prove God doesn't exist, so God exists.' and 'You can't prove God exists, so God doesn't exist.' Just as the absence of evidence of God's existence does not by itself constitute evidence that God doesn't exist, absence of evidence that God doesn't exist does not by itself constitute evidence that God exists. - Curiously, each side in the dispute between atheists and theists often castigates the other for committing this fallacy while committing it itself in the case of its own arguments. Atheists will quite happily point out that the fact science can't show that God doesn't exist isn't evidence of God's existence, and then blithely go on to argue that the fact believers can't prove God exists is evidence that God doesn't exist. And, of course, theists will argue the opposite. - The key phrases, however, are 'only' and 'by itself'. Absence of evidence in support of some statement can, alongside other evidence that positively supports its negation, provide us with some good reason to doubt the truth of that statement. The logician Irving Copi puts it this way: 'The argumentum ad ignorantiam [fallacy] is committed whenever it is argued that a proposition is true simply on the basis that it has not been proven false, or that it is false because it has not been proven true. He adds, A qualification should be made at this point. In some circumstances it can be safely assumed that if a certain event had occurred, evidence of it could be discovered by qualified investigators. In such circumstances it is perfectly reasonable to take the absence of proof of its occurrence despite searching, as positive evidence towards its non-occurrence.'

But even with this qualification it remains the case that absence of evidence is inconclusive and falls short of the requirements of knowledge. It may constitute a reasonable basis for belief, based on an evaluation of relative likelihoods, but not for a claim to knowledge.