>
>I thought I would throw my two cents on this, hope you
>don't mind :).
>
As a worthy opponent you are most heartily welcome. Come, let us toast each
other's bravery and fortitude. Slantha!
>
>This insistence by some that absence of
>evidence is not evidence of absence has always bugged me
>since, for one thing, it seems totally contrary to the
>way science normally works.
>
Not really. Science is based on the idea that you need definitive,
unambiguous evidence that specifically validates or refutes hypotheses. As
I explained to Mark in my latest reply, you can expect one of three basic
results from any experient: positive evidence that verifies the hypothesis,
positive evidence that refutes the hypothesis, or no positive evidence,
which is then called negative evidence. It is negative evidence that the
appeal from ignorance specifies when it refers to "absence of evidence".
Practically speaking, negative evidence is either no evidence whatsoever
(such as from a failed experiment) or evidence too ambiguous to interpret;
it is not positive evidence that refutes the hypothesis (hence producing an
"absence" of positive evidence that would have verified it instead).
>
>If someone proposes a theory
>it is natural to ask that person what the evidence is for
>that theory. But if absence of evidence is not evidence of
>absence then what, pray tell, is the purpose of such a
>question?
>
To test the theory against physical reality, you silly person ;-). Since
any proper theory must have some supporting evidence from the start
(otherwise it is only speculation), this is not what verifies or refutes it.
Instead you use the theory to make predictions about experimental results,
then do the experiments to try to get those results. As such, only positive
evidence that directly and unambiguously verifies or refutes the prediction
will have any impact on the theory. Negative evidence, which by definition
cannot either verify or refute the prediction, can tell you nothing about
the theory. As such, any attempt to use this "absence of evidence" to
refute the theory is a fallacy.
Creationist "theories" tend to be speculation disguised as theory, so they
can be refuted without testing them by revealing their lack of supporting
evidence. This doesn't really involve the appeal from ignorance fallacy,
however, because the speculations-disguised-as-theories don't really
qualify.
>
>Suppose that: (a) there is an absence of evidence
>that I robbed Fred's Bank on High Street yesterday....
>
In other words there is no positive evidence that proves OR disproves that
you robbed that bank.
>
>...and
>(b) there is an absence of evidence that Fred's Bank on
>High Street was robbed by anyone yesterday.
>
In other words there is no positive evidence that proves OR disproves that
the bank was even robbed.
>
>Should this
>absence of evidence be considered evidence that I did not rob
>Fred's Bank on High Street yesterday?
>
No. At best it constitutes evidence that you both did and did not rob the
bank; at worst it constitutes no evidence at all. Either way, based on all
this negative evidence alone no one can make a claim whether a robbery even
occurred, much less whether you were the robber. In other words, the
evidence tells us that we can neither confirm nor deny that a robbery took
place and can neither confirm nor deny that the culprit was Brian Harper.
Practically speaking, however, the end result is the same as if the absence
of evidence were evidence of absence.
>
>Or, more in context,
>does the absence of evidence for a global flood indicate
>evidence that such a flood never occurred?
>
No, because a global flood is not refuted by a lack of positive evidence
that verifies it (i.e., negative evidence), but by the presence of positive
evidence that refutes it. Again, however, the practical results are the
same, hence the confusion.
>
>Yes, there are exceptions of course. Primary seems to be
>pointing to a lack of evidence where there are good reasons
>to say that no evidence of this type should be found. Whether
>this applies wrt pre-Cambrian fossils I can't say. Perhaps.
>
It does in some cases; I believe it would apply in most, but there should
still be some good fossil-bearing strata left to explore in any case.
Kevin L. O'Brien