From Bob Clark (931214.1715 EST)

Bill Powers (931209.0600 MST)

At the end of your lengthy post, you add a comment:

I think we have been overlooking something about "closed loop"
systems. It is not just the closure of the loop that matters. What
matters is that it is a _unidirectional_ closed loop, at least at
one point.

I agree that this is a very important matter. But there are
mechanical systems that have some "rectifying" aspects-- most
familiar in electronics, but also found in purely mechanical systems.
A ratchet comes to mind. Friction can be asymmetric. So
unidirectionality is not a _distinguishing_ characteristic of control
systems -- but it is necessary.

As you point out:

Sensing the state of the controlled variable _does not affect the
state of the controlled variable_ (to any meaningful degree). When
the error signal creates an output from an effector, physically
disturbing that output does not affect the error signal backward,
through the output function.

Interestingly, when I looked up Comparative Neuroanatomy, I found an
organism - Coelenterate, I think -- with "primitive" neurons that
were bidirectional! They seem to operate to produce cooperative
contractions. The control systems they do have seem to be limited to
be biochemical.

Regards, Bob Clark