[From Rick Marken (970222.1120)]
Bill Powers (970222.0843 MST) --
I'm afraid that the term "computing output" has become a code
expression in PCT that threatens to substitute for thought.
Thanks for another inspired post.
I just want to add somthing that I tried to suggest yesterday, viz.,
that the term "model based control" has also become a code expression
that threatens to substitute for thought. What the expresssion was
originally meant to refer to was a particular control architecture:
one in which an adaptive mechanism is used to tune an "internal
model" of the environmental function that connects system output
to plant output. The model doesn't control a perception of plant output
(as does the PCT model); rather, it controls a modeled version of what
this perception _would be_ if the current version of the "internal
model" were a correct representation of the environmental function.
This narrow referent of "model based control" has been expanded (in
usage) to include many examples of good old fashioned control of
perception. For example, just yesterday Martin used the term to
refer to control of a perception relative to a variable reference;
the variable reference was taken to be a "model" of the temporal changes
of a variable in the environment. I also recall uses of the
term "model based control" to refer to control of an imagined perception
(as described in B:CP. p. 222); again, the imagined perception was taken
to be a "model" of some possible real perception. I have even heard the
term "model based control" used to refer to good old, plain vanilla
control of perception; in this case the controlled perception itself is
taken to be a "model" of some objective state of affairs (such as the
speed at which the car is moving down the road).
All of these uses of of the term "model based control" turn on the
notion that a "model" is a representation of something else. In the
"model based control architecture" the model is a representation of the
actual environmental (plant) function. In control of imagination the
imagination is a representation of past perceptions. In control
of perception the perception is a representation of some external state
of affairs. So all these uses of "model based control" are perfectly
appropriate-- just as it is perfectly appropriate to say that the PCT
model "computes output".
I don't think we going to make any progress toward understanding the
real differences between PCT and other explanations of behavior until we
can agree on what we are referring to when we say things like "computed
output" and "model based control". I think Bill Powers has been very
clear about what he means by both terms and I agree completely with his
meanings. I hope some others are willing to
agree to these meanings as well.
Hopefully
Rick