Who is the postivist?

Richard Zander bryo at COMMTECH.NET
Tue Dec 9 09:19:32 CST 1997

Tom DiBenedetto wrote:
>  Richard Zander wrote:
> > Parsimony eliminates grossly unreasonable
> >trees, maximum likelihood eliminates grossly improbable trees.
> I disagree with this. Parsimony selects the tree which represents the
> set of least falsified grouping hypotheses.

That's hypothetico-deductivism. "Wishful thinking subject to correction"
according to Quine. I rather assume like produces like unless
macroevolution is demonstrated, and I see parsimony analysis eliminating
relationships between grossly dissimilar terminal taxa, and using state
changes as a distance measure (patristic). That's okay. In most cases
you don't even need a computer to come up with a general classification
that assumes your version of evolutionary direction (selection of
outgroup). But "reconstruction"?

> >I don't see a way to select one tree as a
> >reconstruction. Every elimination from the set of reasonable trees is an
> >ad hoc assumption of no convergence.
> "No convergence" is not an ad hoc hypothesis. An ad hoc hypothesis is
> one which addresses a particular case and seeks to explain why it is
> incompatable with a more general hypothesis. The general hypothesis
> is homology. "No convergence" is a confirming explanation; it admits
> that the evidence is explainable by the general hypothesis.

The general hypothesis is that all reasonable trees are Darwinian
explanations of evolution. Your general hypothesis is convenient for
selecting one tree from dozens or hundreds. Looks good but the general
hypothesis is wrong to begin with.

> >Realists think there is a truth towards which the best hypothesis
> >converges, anti-realists think truth is limited to the phenomena (data
> >set).
> Realists think there is a truth which we are trying to describe. It
> is not necessary to assert that the best hypothesis is actually
> converging on the truth, although we obviously try to develop our
> science in such a way such that it does.
> Anti-realists (by your definition) seem to deny the reality of taxa.
> Once again, if that is your position, then why bother with
> systematics?

Pragmatically, a classification based on overall similarity of taxa
groups organisms such that we can best predict things about them.
Phenetics and cladistics and max likelihood trees are generally alike,
and that's encouraging. I would use the tree or dendrogram or cluster
that predicts best with external verification. Until that verification,
which is not a measure of truth but of probability or expectation of
continued usefulness, is to be had, no one's method impresses me with
the ability to select one tree.

> > What
> >is important is that Realists have lower standards for hypotheses than
> >Antirealists. As long as you can demonstrate an increase in probability
> >(lower falsifiability, whatever), you can select the best example as
> >best explanation and therefore asseverate one tree as "approximately the
> >truth." Even when the one tree has a low probability of being the truth.
> >This is scientifically unconscionable.
> But that is totally dependant on your calculation of probability. How
> do you calculate that? Do you believe that life has had a single
> history? Given that there are 30-80 million species, and more
> possible phylogenies than quarks in the universe, how can any
> phylogeny have a probability more than a smidgen above zero? And yet
> one is true.

Again you assume that the theory has to be the same as Truth, whatever
that is. Given lots of (sometimes outrageous) assumptions, statistical
phylogeneticists do come up with trees that have probabilities attached
to them. I know of a study that presented a tree selected from 10^40
trees. The tree is reasonable, but so are a number of other trees
slightly less probable. The test is usefulness in a scientific context
(a la Piercian pragmatism) and it should be external.


Richard H. Zander, Buffalo Museum of Science
1020 Humboldt Pkwy, Buffalo, NY 14211 USA bryo at commtech.net

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