[From Rick Marken (960223.2130)]
Chris Cherpas (960223.1327 PT) --
How can references _ever_ match inputs?
I think it would help if you take a look at the operation of
a simple control model. There must be one in DEMO1. You are
already having a problem if you think that control systems
match references to inputs; in fact, control systems act to
force inputs (perceptual variables) into a match with references.
Here's why I ask: PCT makes the claim that specific responses
can't possibly be what is learned since you're always in different
orientations/positions when you act, so the same set of muscle
tensions won't work for a changing environment.
Basically correct. But you forgot to mention the reason why the
responses vary; to control a perception. Responses vary in order
to keep a perceptual input variable in a reference state.
But what about the "stimulus" side? Don't we see, hear, feel, etc.,
everything from all sorts of orientations, viewpoints, angles, etc.?
Yes. A stimulus is an external event (a disturbance in PCT language);
the perceptual representation of that disturbance depends on what
the organism is "doing" at any instant; its orientation, viewpoint
and angle with respect to the stimulus (disturbance). This fact is
captured in PCT by saying that p = d + o; the perceptual representation
(p) of an external variable (d) depends on the value of that variable
(how it moves relative to the organism, say) and how the organism acts
(o) with respect to that variable (how the organism moves relative to
the stimulus, say).
Therefore, how can references possibly work under such variable
conditions? How can any inputs match a reference if the world
is so variable?
It is the _representation_ of the world that you were referring
to as being variable; that representation (p) is, indeed, variable,
as is the world itself (d) and our orientation with respect to it (o).
For example, suppose that the "stimulus" (d) we are talking about is a
pretty girl (if I seem to be obsessed with pretty girls lately it
may be because I turn 50 next week;-)). Let's say that the reference
for the image (p) of this girl is "centered in the field of vision".
If we keep our head stationary (o = constant) while the girl walks by,
then, as the stimulus changes (d varies) p varies (p = d + constant).
If we move our head while the girl remains stationary (d = constant)
then, again, the perception varies, but this time because our own output
varies (p = constant + o).
Now suppose that both d and o vary; the girl moves and our head
moves. In this case, by appropriate movement of the head, we can
keep the image of the girl (p) in the reference state (centered);
that is, we can vary our head movement (o) so that d + o = reference.
The magic of control is that the control system is able to vary o
appropriately to keep p = o + d = reference even though the only thing
the system itself can see is the result of varying o -- which is the
reference value of p. The control system cannot perceive d; it only
perceives o + d.
what PCT principle or slogan makes this possible?
Behavior: The control of perception.
maybe that's the whole point: since inputs don't match references
exactly, we keep behaving until they do.
By George, I think you've got it! 
Still, how can actions (re)create so precisely whatever it is that
will make inputs match references?
That is the $64,000 question. The envelope please, Mr. Powers.
The answer is "by being the output variable in a closed, negative
feedback loop". In this loop, actions (o) vary (over time) precisely
as as necessary to compensate for changes in the "stimulus" (d) and
keep p = reference. The best way I've found to understand how this
works is by observing the operation of a simple control system --
over and over and over and...
I'm going around in circles.
Always a good sign when you are learning PCT;-)
Can anybody get what I'm asking, and, if so, help clarify my thinking?
Was this any help?
Best
Rick