On 3/13/2012 1:00 AM, Warp wrote:
>    What makes the difference? Why is this evidence for evolution valid and
> proper, but eg. the existence of life or alleged miracles for the existence
> of a god isn't?

This is just off of the top of my head, so it may not coincide with the 
"accepted" answer to your question, if such a thing even exists.  It's 
also not necessarily very well thought out yet.

Anyway, It seems that something related to your question is the 
oft-mentioned quip that evidence can evidence can only disprove a 
theory, but never prove it.  In what sense can it be said that we have 
"evidence for" anything then?  Obviously there must be some meaning to 
the phrase, since otherwise a theory which says "anything is possible" 
would be regarded as an ideal theory, since it's obviously never been 
falsified.

The most obvious answer I can see is to consider the question from a 
statistical perspective rather than a perspective of pure deductive 
logic.  In this viewpoint the predictions yielded by a theory would be 
interpreted as a probability distribution over (a subset of) observable 
events.  The goal of a good theory should then be to make predictions 
which match the actual observed probabilities (and possibly satisfy some 
aesthetic criteria).

In addition, instead of talking about the merits of a theory by itself, 
I'll switch to the view that, strictly speaking, evidence can only be 
used to differentiate between different competing theories.  In this 
view there are, colloquially speaking, *two* ways in which evidence 
might support theory A over theory B.  Firstly, as in the logical case, 
you could observe something which theory B predicts is very unlikely or 
impossible, but which theory A give higher probability to.  Secondly, 
you might find that theory B predicts more events than are actually 
observed, whereas theory A gives low probability to unobserved events.

So what of your question then?  Fossil evidence can support a theory 
like evolution in the case where evolution predicts a high likelihood of 
some events (like your fish example) which competing theories don't 
ascribe any particularly high probability to.  For instance your basic 
intelligent design argument would probably give finding such a fish the 
same probability as finding countless other sorts of things which 
weren't observed.

Of course strictly speaking we don't exactly have evidence "for" 
evolution (or whatever theory you like), just for evolution in favor of 
our ideas for currently competing theories.  If some future theory makes 
even more precise and accurate predictions than evolution, then it's 
probable that this theory would also give a high probability to your 
fish example and thus the fossil wouldn't give evidence to evolution in 
favor of this hypothetical new theory.

As an aside, the fact that you mention a case where the evidence was 
found *after* the theory was formulated is interesting.  Strictly 
speaking you'd think it wouldn't matter when the evidence was found, 
just what that evidence was.  Nevertheless, such examples are extremely 
useful because they help assure us that we haven't "cheated" and created 
a theory which is little more than rephrasing known evidence in a 
different way, as it's impossible to bake predictions into a theory if 
you don't know what these predictions should be.  So I see this as more 
a vital psychological tool rather than something directly related to the 
theories themselves.

Does this make sense?  I ask genuinely since I haven't really though 
much about it.  I also suspect that you could draw some more rigorous 
ideas about the "aesthetic criteria" a theory should preferably satisfy 
by looking at information theory, but I don't care to give it a go at 
the moment.

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