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evolution-digest Tuesday, February 23 1999 Volume 01 : Number 1314
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Date: Mon, 22 Feb 1999 15:11:54 -0800
From: Brian D Harper <bharper@postbox.acs.ohio-state.edu>
Subject: Re: Cambrian Explosion
At 08:20 PM 2/16/99 -0700, Kevin wrote:
>Brian wrote:
>
>>
>>I thought I would throw my two cents on this, hope you
>>don't mind :).
>>
>
KO:==
>As a worthy opponent you are most heartily welcome. Come, let us toast each
>other's bravery and fortitude. Slantha!
>
This sounds like something one hears just before being skewered ;-).
>>
>>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.
>>
>
KO:==
>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).
>
I'm afraid I'm getting confused. Perhaps I'm slow :). It always
helps me to consider examples, preferably different from those
under dispute :)
When I think of lack of evidence in a scientific setting
I'm thinking mainly of two possibilities:
1) There is a particular piece of evidence which would really
bolster a theory were it available. Unfortunately, it is not.
2) The theory makes a specific prediction that such and such
evidence should be found in such and such a fashion. But this
evidence is not found.
How would you define these two "lack of evidence" situations
using your terminology?
It might help to think of two concrete examples for each.
1) Newton's first law says a particle moving in a straight
line at constant speed will continue to do so *forever* unless
acted on by a force. Direct evidence for this would be the
observation of a particle moving in a straight line at
constant speed in absence of forces. I believe evidence of
this type is lacking. Since the lack of evidence arises because
the critical experiment cannot be performed we should, I think,
be neutral regarding this particular lack.
2) For an example of the second type we'll propose an indirect
test of the law of inertia and a direct test of the inverse
square law. Specifically we propose to measure how far the
moon "falls" in some time interval. By "fall" I mean more
precisely the distance that the moon deviates from its natural
(inertial) motion. The experimental phase of our test was
accomplished by astronomers before Newton published his universal
law. Thus, all Newton had to do was see if he could predict the
results. Turns out he failed to do so, resulting in a five year
delay in publication. Apparently Newton thought that the absence
of this evidence was a critical blow to his theory. Later someone
(not Newton) repeated the measurements finding the initial results
to be in error. When Newton obtained the revised calculations he
found they matched his predictions beautifully. He published.
>>
>>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?
>>
>
KO:==
>To test the theory against physical reality, you silly person ;-).
Exactly. And if the person replies that evidence is lacking, how
well does his theory score on this test?
KO:===
>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.
I fear we may be talking past each other as I find the above to be
an affirmation of my point :). If a proper theory must have some
supporting evidence then clearly the absence of same would be detrimental
to that theory. Once we establish this then the rest seems a matter
of degrees. Some theories have more evidence then others. These are
more firmly established. Why?
KO:==
>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.
>
No. IOW there is no evidence I robbed the bank. My fingerprints
weren't found, no eyewitness placed me in the bank. etc.
>>
>>...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.
>
No. Surveillance cameras do not report suspicious activities. No teller
reports having been robbed. Records show that no money is missing,
etc.
>>
>>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.
>
Really? I'm afraid I'm just not getting it. Suppose we take Last
Thursdayism or Invisible Pink Unicornism?
>>
>>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
>
>
>
Brian Harper
Associate Professor
Applied Mechanics
The Ohio State University
"He who establishes his arguments
by noise and command shows that
reason is weak" -- Montaigne
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End of evolution-digest V1 #1314
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