NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/bit/listserv/csgl/1959

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Date:         Wed, 23 Dec 1992 15:13:37 EST
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Subject: Re: Descriptive/generative; info theory; applying PCT
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[Martin Taylor 921223 14:40]
(Bill Powers 921223.0915)

>>But I'm equally impressed with the argument from evolution and
>>other more abstract arguments of necessity and possibility.
>>They all support one another.
>
>The problem with arguments from evolution and other abstract
>arguments is that while they may apply to a particular case, they
>may also turn out to apply to counterfactual cases. They may
>contain flaws that don't show up in the factual case (for
>instance, flaws in internal logic), but those flaws can make the
>generalized explanations specious, even though they "support" a
>more concrete analysis.

What I'm talking about is sometimes called "converging operations."
Or it was when I was in graduate school.  If looking at a problem from
several different viewpoints gets the same answer each time, or if an
answer turns out to be in common among many from each of the various
viewpoints, then that answer becomes much more credible than if it is
precisely determined from only one viewpoint.

I doubt that I would buy PCT simply because an evolutionary analysis
suggested that hierarchic control systems would have enhanced stability
and hence would be observed preferentially among living organisms.  But
I would be much less likely to buy PCT if the evolutionary argument went
the other way.  It helps the credibility of PCT a great deal that organisms
that are organized as control hierarchies are more survivable than
organisms with other proposed structures (e.g. stimulus-reactors or
simple planners).

>Descriptive models, however, are not very sensitive to
>experimental test, some not at all. If they're cleverly put,
>their predictions can remain true no matter what the outcome of
>an experiment. So they are capable of "supporting" completely
>contradictory generative models. This is not really support at
>all. It is merely agreement.

Yes, the degree of support depends on where the competition is.  If
the descriptive model supports two contradictory generative models, it
supports neither, but it might support that class of models (the gaphroomus
could be black or white) against a different class (the gaphroomus mustn't
be spectrally coloured).  The importance of agreement depends on the prior
likelihood of disagreement.

This aspect of converging operations applies to your critique of Gary.  If
PCT usefully suggests ways of assisting people, and those ways work reasonably
often (not necessarily always), then that success also argues that PCT is
a valuable way of looking at the psychological world.  It doesn't prove
anything, but it does reinforce the idea that PCT is worth following up
the levels hierarchy, and is not restricted to tracking tasks with
measurable errors.  You have also provided examples at the level of
personality (self-image) that make the same point.
---------------
Occam's razor deals with the amount of specification needed in order to
desribe phenomena.  If one descriptive method can be used over a wider
range of phenomena than another, or if it describes phenomena more precisely
within a given range, it is to be preferred.  PCT does both.  It substitutes
one descriptive method for a variety of topic-specific theories, and in
the application area of each theory it makes more precise descriptions.

To me, a model is only a concise way of making descriptions.  A generative
model, as you put it, is distinct from a descriptive theory.  To me, it
is not distinct.  It only uses a few more parameters to make a much better
description, and if that description is accurate, Occam's razor says that
it is preferred.  But if its precision allows the world to say "that's not
right," it could be that the parameters are wrong.  If the parameters can
be changed and still lie within the bounds of "the theory", then the
information contained in the parameter specification has to be included
in the "size" of the description, so the case becomes less clear.
===================

On information theory: Channel capacity is not specifiable from physical
characteristics alone. A physical channel may have many different capacities
simultaneously.  And (as follows from one of your own postings a while back)
you can have continuous information flow without signal flow.

I prefer, I think, to set aside postings on information theory until I can
produce a coherent PCT-oriented tutorial.  That will, I hope, provide a
reasonably good argument as to why PCT and specifically HPCT is, with
high probability, the only viable organization for thermally unstable
organisms (i.e. life) to persist. As both Tom and Rick have said directly,
and you imply, the argument may not turn out to be as strong as that, and
may provide only agreement.  But at least it will be an argument from a
different viewpoint against simple planners and S-R organisms, and provide
a converging operation in support of PCT.

I have a growing suspicion that we have a different philosophy of science,
and that at some point this difference will be the theme of a posting thread.

Martin