Who is the postivist?

Tom DiBenedetto tdib at UMICH.EDU
Sat Dec 6 00:28:06 CST 1997

James Francis Lyons-Weiler wrote:

>        What some see as "modern positivism" can be summarized
>        as follows:
>        A misplaced belief that an inference is scientific, and
>        therefore accurate and precision, simply because it is
>        based on a formal method.

This is of course, hardly a definition of positivism, nor much help
to anyone trying to understand what you are referring to.
Does anyone believe that a method is scientific merely because it is

>       Positivism was adopted by the
>        Vienna circle, but my studies indicate that they merely
>        co-apted the British model of empricism; in so doing, they
>        placed their faith in a single mode of inference, and
>        became modern positivists.

>From my understanding, positivists can be usefully distinguished from
Popperian critical rationalists by the former's adherence to a
verificationist approach to assesing the validity of scientific
theories, as opposed to Popperian falisificationism. In modern
phylogenetics, it is (some) cladists who stress the falsificatinist
approach, whereas the statistical phylogeneticists (those lost in the
Felsenstein zone) seem to be inherently verificationist.

>        A positivist
>        would argue that based on first principle alone, regardless
>        of how well or poorly it may perform in general, MP provides
>        the best estimate _in a particular case_.

You are referring to the uses of maximum parsimony in molecular
"systematics". I think that our discussions have been less than
efficient because I speak of cladistic parsimony, and that is rooted
in a very different approach. I have tried to emphasize the
difference in many of my postings; I am not sure how much success I
have had.

Once again, and very briefly, cladistic parsimony utilizes a
parsimony criterion in the
implementation of the test of congruence. We do not use parsimony as
a test in itself. (Perhaps James is led to his reference to
"parsimonious evolution" because of confusion on this very point). We
begin with hypotheses of character homology. The historical
interpretation of those homologies (they are "the same" because of
descent from a common ancestor) implies a test; true historical
homologies should form a congruent hierarchical pattern with regards
to their distributions amongst taxa. The test of congruence is
implemented by combining the homology hypotheses
to see what sort of a congruent hierarchy exists. Parsimony is a
logical principle which underlies the application of this test; less
than maximally parsimonious solutions are simply inefficient
implementations of the test; they find less congruence than is
actually present in the data.
The result of a cladistic analysis is a set of character homologies
which have been corroborated both through the biological tests they
have endured, and the congruence test. Their logical combination
represents the sum of phylogenetic evidence from character
distributions, and our best overall reconstruction of what the
lineage branching pattern was.
With James's interest in the "truth", in terms of the "windows to the
past", I would suggest investigating Popper's notion of
"verisimilitude"; which is basically the substitute for "truth",
since truth is unknowable. Verisimilitude is a notion of "closeness
to truth" and Popper develops a logical calculus for it. I havent had
time to develop arguments around this concept, but I suspect it will
be useful for justifying

Tom DiBenedetto                 http://www-personal.umich.edu/~tdib/
Fish Division                                   tdib at umich.edu
University of Michigan Museum of Zoology

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