Re: [asa] The Parsimony Principle (aka Occam's Razor) and atheism

Iain had written:
>>> The argument goes something like this. Because the laws of
physics and >>>the theory of evolution can explain, in principle,
everything there is,
>>> including all the complexity of life, etc, there is no need to invoke a
>>> Creator to explain it all. ...

I replied
> > This makes the assumption that the physical universe is all that there
> > is, or at least all that's interesting. Thus, it's circular reasoning.

The laws of physics, evolution, etc. explain physical things. The
claim that this in principle explains all that there is makes the
assumption that physical things are all that there is. This is where
I find circularity in this part of the argument.

> This is because the combination of model + differences from the model
> prediction admit a more compact description of the data than the raw pile of
> data itself. The raw pile can't tell you anything about the nature of the
> data, what laws are obeyed etc. But the fact that it approximately fits,
> say, an exponential curve tells you quite a lot.

True, but you still need the raw pile of data to get to the model and
differences. Thus, the total amount of relevant stuff is larger with
a model than without. The real problem here is one of demarcation.
What exactly do we want to make simple? As a rule in science we are
seeking to simultaneously maximize the breadth of explanations
(minimize the number of models), minimize the complexity of the
models, and maximize the accuracy and precision of the models. The
raw data versus model example minimizes the complexity and number of
models but gives no breadth of explanation whatsoever. Claiming model
with God is less parsimonious than model without God make a similar
error-the model with God explains the spiritual realm as well as the
physical and so is a broader explanation but it does not improve our
ability to calculate physical parameters.

> > Another example of misuse of parsimony comes from some advocates of a
> > particular approach (called maximum parsimony) to the generation of
> > evolutionary trees. Some lose sight of the fact that the goal is to
> > represent the data and advocate more parsimonious against less
> > parsimonious (at least by the particular metric, which is not entirely
> > appropriate as a comparison) but reflecting the uncertainty in the
> > data.
>
> Interesting - I was aware of this principle, but didn't know it was
> considered a misuse. On the face of it, it does fit my expectation of a
> reasonable use of it - trying to decide between different postulated
> evolutionary paths. The probabilistic interpretation of Occam's razor
> implies that the simplest explanation is the most probable (though that, of
> course doesn't rule out the brute possibility that a less probable path may
> be the one that actually occurred).
>

Maximum parsimony is widely accepted, although a simplistic version is
known to fail under certain conditions (e.g., when the probability of
DNA random mutations hitting on the same base rivals the probability
of base similarity being inherited). The problem I alluded to is in
trying to apply it inappropriately because it's not really the issue
at hand. An example is when parsimony analysis yields multiple
equally parsimonious trees. This result could result from factors
such as inadequate data, very rapid evolutionary radiation,
non-treelike evolutionary patterns, etc. It's conventional to show
this by using a consensus tree. Areas unresolved in the tree show
where the data do not provide enough resolution to decide. However,
folks overly carried away with being parsimonious have claimed that
since the consensus tree itself is typically less parsimonious than
any one of the trees generated by the analysis, you should pick one of
those trees instead. This ignores the fact that the consensus tree
tells you about the agreement and disagreement between the most
parsimonious trees. Of course, there's a long tradition of
systematists chosing the wrong tree (Gen. 3).

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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