[From Rick Marken (960214.0845)]
Hans Blom (960214) --
I did The Test in that I picked up the particle, put it somewhere else, and
then let it go again. It returned to where it seemed to "want" to go again,
if I may use those words -- i.e. it resumed climbing the gradient.
The fact that you were able to pick up the particle and put it somewhere else
suggests that the particle is not controlling its position; the disturbance
to position ("picking up the particle and putting it somewhere else") is
fully effective. The fact that the particle resumes its climb up the gradient
after you remove the disturbance ("let it go") is not an indication of
control; it is an indication that other forces were also acting on the
particle while the disturbance force predominated.
According to your version version of The Test, a ball rolling down an
inclined plane would qualify as a control system. When I pick up the ball,
move it to another point on the plane and let it go, the ball resumes its
climb down the plane. This is exactly what you did to Test for control in the
Brownian motion study. I presume, then, that you would conclude that the ball
(like the Brownian particle) is controlling its position.
I guess that even without formally performing such a Test [using a
continuously varying disturbance], you would know the answer already --
since you are familiar with the underlying mechanics: in all cases, the
particle will still try to climb the gradient, in addition to being subject
to some externally applied force/disturbance.
Try it. I think you will see that continuous temporal variations in the
applied force/disturbance will be completely effective, resulting in
proportional continuous temporal variations in partical position that are
highly correlated with variations in the disturbance. The position of a
Brownian particle will clearly and unambiguously fail the Test for the
Controlled Variable. The same is true of the ball rolling down a plane. Just
push the ball up and down the plane; these pushes will be fully effective;
the ball will move up and down the plane with your pushes; the position of
the ball is not controlled.
Why do you stress _continuous_ disturbance so much?
Because what matters in control is the nearly simultanenous compensation for
disturbance by system output. You and Martin keep taking about disturbances
that come and go, and you look at what happens to the putative controlled
variable after the disturbance has gone. This is not the way to test for
control; but it is precisely the way to fool yourselves into thinking that
the behavior of an equilibrium system is like that of a control system.
I love to investigate what some others call superficialities ;-).
That's obvious. You are defintely not alone. Note the apparent success of the
Santa Fe Institute -- dedicated (unintentionally, I presume) to the study of
superficial similarilties between equilibrium and living systems.
Control _does_ refer to something -- to a range of phenomena that have to do
with stabilization in the face of disturbances -- homeostasis.
Wrong. Control refers to one, precisely defined phenomenon. You have your
own reasons for wanting to conflate control and equilibrium (stability)
phenomena. But, as you say, that's your problem. Could you please stop
trying to make it other people's problem too.
Hans Blom (960214g) --
Wouldn't you have to compare with how the variable would vary if it
were NOT under control? ...don't you need some theory before you can
generate a prediction of what it would look like?
Yes. Good point.
PCT assumes that variables in the environment behave according to the "laws"
of basic physical theory; the "null hypothesis" of The Test is that the
behavior of the variable under study is the result of good old cause-effect
processes (Newton's laws). When it appears that variables are behaving in a
way that is not consistent with physical law (girders moving up or down at a
constant velocity, objects moving when there is no obvious external force,
entitites formed into remarkable organizations like processor chips, etc) we
Test to see whether these variables are under control. And we don't reject
the null hypothesis unless we are VERY sure that there is no cause-effect
explanation of any failure to find an effect of disturbance. This was Bill
Powers' (960214.0100 MST) point when he said:
Also, I once again remind everyone that the Test does not consist only
of the application of disturbances and observations of perturbations.
The lack of an effect of an applied disturbance could be due to an
incomplete understanding of the physical situation, or some other system
beside the one you have in mind might be doing the controlling. To
complete the Test, you have to verify that the appropriate input and
output connections are present, so you know which system is doing the
controlling and what the input and output pathways are. The
identification of the input pathway is done by interrupting it and
demonstrating that control is lost (under Hans' model, this might take
some time).
DOING a test is not difficult. Thinking of a good one is...
Yes. Now you should have a much better idea of how to Test for controlled
variables.
Have a nice day (I certainly am; I got my honey a great Valentine's day
present)
Rick