emergence and parsimony - modeling theory!

[Chris Landauer]

Sven N. Thommesen wrote:
 > Epstein and Axtell suggest that we are engaged in 'generative science': we
 > are looking for micro-specifications (i.e. agent behaviors) that are
 > capable of generating a given macro behavior.
...
 > So, should we not also require that our behavior rules at the micro level
 > be 'plausible' in some sense?

"glen e. p. ropella" <address@hidden> then answered:
        In most cases, the map between this micro behavior and the macro
        behavior is totally opaque.

yup, exactly so

"glen e. p. ropella" <address@hidden> also answered:
        If it's *not* a real system, then you've only got two choices: develop
        the micro behavior into a formal system and proceed via standard
        proofs, or integrate the system forward in time and measure the macro
        behavior.

nonsense - many choices exist, always, whether or not the system is *real*

the basic question is about the relationship between the locally-defined
mechanisms and the globally observable phenomena - the micro and macro levels
of the second message

we have many observable "macro" phenomena - we hope ("expect" i think is too
strong a word) that we can devise corresponding "micro" mechanisms that
explain such macro behavior, so we can hope to change the one and see a
corresponding change in the other, but there still are not very many useful
tools for examining the consequences of "micro" assumptions

the practical challenge of emergent behavior is to understand what makes it
happen, i.e., what makes the micro behavior lead to the observable macro
behavior, and the relationship between them

the theoretical challenge of emergent behavior is to characterize the kinds of
relationships that can occur, and to devise ways to compute macro from micro
more quickly than by using simulation, and to suggest choices for or classes
of micro from desired or observed macro, to reduce the difficulty of inventing
or explaining interesting systems

finally, we note that parsimony simply does not apply (Occam's razor is
usually the wrong choice for reasoning about biological systems) - one of the
well-known facts about biological systems is that there are many "historical
artifacts", whose existence depends on how easily is was to change from
something that _already_ existed, not on how easy it was or is to invent or
describe the concept from first principles

more late,r
cal


Dr. Christopher Landauer
Advanced Programs Operations
National Systems Group
The Aerospace Corporation
The Hallmark Building, Suite 187
13873 Park Center Road, Herndon, Virginia 22071
e-mail: address@hidden
Phone: (703) 318-1666, FAX: (703) 318-5409