# System Dynamics

From [Marc Abrams (2005.12.06.1555)]

I’d like address an area that I believe a lot of misunderstanding and a lack of real knowledge has , I believe hurt PCT and the modeling efforts of many.

Most people, even current modelers in SD believe that SD is all about differential equations, derivatives and integrals but I am going to show you that this is a huge mistake.

Jay Forrester the developer of SD is an electrical engineer and besides developing SD worked on the first core memory for computers at MIT, but he was basically a servo-mechanism engineer, so control systems was a big part of his background.

In 1960 he wrote his second book, Principles of Systems, and in it he provided an introduction to system dynamics and modeling.

SD basically has the three modeling components. A level, a rate, and auxiliary variables. That is it, and most folks think that the ‘rate’ component is nothing more than the derivative; WRONG. I’ll let Jay explain what it is himself;

Pgs 4-14 -15

We turn now to the components of a rate equation–the sub-substructure
of a system. Four concepts are to be found within the
rate equation ( that is , a policy statement):

1. A goal
2. An observed condition of the system
3. A way to express the discrepancy between
goal and observed condition
4. A statement of how action is to be based
on the discrepancy.

Pg 4-16;

Pg 4-16;

···

Principle 4.4-1. Goal, observation, discrepancy,
and action–system
sub-substructure.

A policy or rate equation recognizes a
local goal toward which that decision point
strives , compares the goal with the apparent
system condition to detect a discrepancy,
and uses the discrepancy to guide action.

Now, I don’t know about you, but this all looks very much like a control process to me, how about you? And the description’s above and below the diagram with the equation on top pretty much seals the deal

Yes folks, SD was intended to be used to model control systems. How about that. Not only that, but most current SD modelers have NO CLUE that this is in fact the case.

Jay even went out of his way to dismiss the use of differential equations, again in his own words;

Pg 6-11;

6.4 Differential Equations a Regression

Integration (or accumulation) shifts the time-character of
action, produces delays between action streams, and creates the
dynamic behavior in systems. Integration occurs naturally in both
the physical and biological worlds. The integration processes of
the real world is represented in our models by the level equations.
The level equations are first - order difference equations,
and, with a short enough solution interval , are entirely equivalent to the continuous process of integration.
But, those who have already studied some of the mathematics
of dynamic systems have almost certainly done so in terms of
differential equations rather than integral equations. Most of
the mathematics of systems, developed originally in the physical
sciences and engineering, has been cast in terms of differential
equations. Even so , the differential equation view of systems
seems to mislead many students and leaves them without a strong
linkage between the real world and the world of mathematics. For
those who have been introduced to systems by way of differential
equations, this section may explain the integral equation
emphasis of this book.

studied mathematics through differential equations.

Pg 6-12;

Formulation of systems in terms of differential equations obscure
for many students the direction of causality within systems, or , even
worse, creates intuitive feelings of reversed cause and effect relationships. For example, consider the relationships between position,
velocity, and acceleration. Thinking of velocity as the slope, or derivative , of the position versus time curve can suggest that the
changing position is responsible for the velocity rather than the other
way. The direction of causality stands more clearly when the system
description starts with the force that causes the acceleration, integrates
acceleration to produce velocity, and integrates velocity to produce position.
Representing a system in terms of integral equations gives a more
immediate and evident equivalence between the model and the real
system. Such emphasis on integration is plausible when one notes
that all the processes of nature are the processes of integration.
Nowhere in natural processes does differentiation take place. True
differentiation would depend on measuring a velocity instantaneously.
But such is not possible. All natural and man-made devices that seem
to measure velocity actually operate by a process of integration that
in some sense measures the difference between past and present
positions.
The “differential analyzer” illuminates the inescapable nature
of the process of integration. The differential analyzer is a
mechanical or electrical device for generating behavior in accordance
with a set of differential equations. But the differential analyzer
is built of integrators and, before it can be used, the differential
equations must be converted to integral equations.
Except for the reorientation imposed on those whose experience
has been entirely with differential equations, the quickest and
simplest route to understanding dynamic systems seems to be through
models depending on integration , avoiding completely the rather
artificial concept of differentiation .

So as you can see, not only is SD perfectly suited for modeling control processes, the nature of SD modeling has been grossly misunderstood.

You can of course use differential equations to model in SD, but in doing so you tend to formulate the model in a ‘cause-effect’ manner, getting away from the concepts of goals and discrepancies. This is not a ‘bad’ thing, it just provides what I think is an inferior view of the problem.

I first realized all this about six months ago.

About four years ago I helped set up a meeting between Jay Forrester and Bill Powers. George Richardson and Bob Eberlein were also in attendance. If I knew then what I know now I would have been at that meeting as well because as it turned out, neither Forrester nor Powers fully understood the others position, nor how one integrated with the other.

I asked Bob shortly afterward how the meeting went and he said it went well and that he liked Bill but found no differences in SD and PCT. Only that PCT was more ‘detailed’. He said the mathematics were identical.

I couldn’t for the life of me figure out why Bill was unable to communicate to Jay his ideas and the differences between the two, and then I realized that Bill did not know SD nor did he understand what the modeling paradigm was.

Jay on the other hand has no clue about the control of perception and SD models simply show human activity as black boxes embedded in larger systems. There are only a handful of SD modelers who work in psychology and they either utilize a behavioral or cognitive perspective.

I know Jeff Vancouver is working with SD, and Jeff if you are reading this I am working with Ralph Levine. He’d love to hear from you, give him a call.

Bill published a paper in the SD journal a number of years ago, but lost any hope of gaining any interest in PCT when he did not present his ideas in a way SD modelers would find useful, and that would be in an SD model.

So you can cry all you want about not getting any respect from anyone, but if you don’t give or show any respect, why be surprised when none is reciprocated.

I strongly recommend that anyone who might be interested in modeling control processes look into SD. Yes, you can model in practically any programming language but SD makes it so much more accessible.

You might want to get your hands on Principles of Systems from Pegasus Communications for \$40.00. It is both a book and a workbook and is terrific, even if you have no intention of actually modeling but would like to know the nature of the method.

If anyone is interested in learning how to model in SD I can be talked into setting up a little intro. I am not currently a master model builder but I will

be one day and I’m good enough to do what I need to do with it.

Anyone interested?

Regards,

Marc

[From Rick Marken (2005.12.07.0840)]

Marc Abrams (2005.12.06.1555)

I'd like address an area that I believe a lot of misunderstanding and a lack
of real knowledge has , I believe hurt PCT and the modeling efforts of many.
...

So as you can see, not only is SD perfectly suited for modeling control
processes, the nature of SD modeling has been grossly misunderstood.

...

About four years ago I helped set up a meeting between Jay Forrester and Bill
Powers. George Richardson and Bob Eberlein were also in attendance...

I asked Bob shortly afterward how the meeting went and he said it went well
and that he liked Bill but found no differences in SD and PCT. Only that PCT
was more 'detailed'. He said the mathematics were identical.

PCT is based on control theory. So the equations of PCT are exactly the same
as the equations of control theory. So if SD models are control models then
the SD equations will be identical to the PCT equations.

What distinguishes PCT from other approaches to behavior based on control
theory (and the great insight and achievement of William T. Powers) is in
how PCT maps the control model to behavior. The PCT mapping, which puts
reference signals inside the organism and explicitly recognizes these
reference signals as specifications for the state of perceptual inputs,
distinguishes PCT from all other applications of control theory to
understanding behavior. I think a reasonably clear description of the
difference between the PCT and all other applications of control theory to
behavior is give in my review of the book "Control Theory for Humans" which
is a textbook on the application of control theory to behavior. My review is
at

One final word. As far as I'm concerned, the easiest way to tell the
difference between PCT and other applications of control theory to behavior
is to look at how behavior is studied. Students of behavior who use non-PCT
control models study behavior using conventional IV-DV methodology. Students
of behavior who use the PCT control model study behavior using the test for
the controlled variable (TCV). I would consider SD and PCT to be equivalent
approaches to understanding behavior if researchers who study behavior based
on the SD model use the TCV as their basic approach to understanding
behavior.

Best regards

Rick

···

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From [Marc Abrams (2005.12.07.1234)]

In a message dated 12/7/2005 11:46:20 A.M. Eastern Standard Time, marken@MINDREADINGS.COM writes:

···

[From Rick Marken (2005.12.07.0840)]

Just as might have a expected. Rick, you are a phoney and an ignorant one with hubris to boot. An extremely lethal combination.

System Dynamics is a modeling method. It is NOT a theory of human behavior.

You need a much better pair of reading glasses because as I pointed out in my post SD does not utilize the PCT theory although both utilize negative feedback loops.

The reason I posted that yesterday was to try to show that SD was a perfect medium for modeling PCT or any other control in but you really don’t and can’t seem to learn from past mistakes you make because you keep on making the same claims.

You do not know system dynamics nor will you ever, and that is a shame for all concerned. You are so consumed keeping any knowledge out of your mind that might infringe upon your loyalty to the PCT theory that I might be better off talking to the Pope about converting to Judaism than getting you to look at anything that touches on PCT.

There are certain people who simply cannot deal with certain things not be so, and for you my dear man it is PCT that has been sanctified, and I’m extremely sorry to see that.

PCT is based on control theory. So the equations of PCT are exactly the same
as the equations of control theory. So if SD models are control models then
the SD equations will be identical to the PCT equations.

No Rick, SD and PCT are NOT identical. SD utilizes negative feedback, and also utilizes positive feedback. The various SD software packages like Vensim, iThink, and Powersim offer many other options for mathematical modeling as well.

What distinguishes PCT from other approaches to behavior based on control
theory (and the great insight and achievement of William T. Powers) is in
how PCT maps the control model to behavior. The PCT mapping, which puts
reference signals inside the organism and explicitly recognizes these
reference signals as specifications for the state of perceptual inputs,
distinguishes PCT from all other applications of control theory to
understanding behavior. I think a reasonably clear description of the
difference between the PCT and all other applications of control theory to
behavior is give in my review of the book “Control Theory for Humans” which
is a textbook on the application of control theory to behavior. My review is
at

What does any of this have to do with my posting? What strawman are you beating off? You are in very sad shape and I feel for you, to feel so threatened by a non-existent ‘enemy’ is the worst kind of fear to walk around with.

One final word. As far as I’m concerned, the easiest way to tell the
difference between PCT and other applications of control theory to behavior
is to look at how behavior is studied. Students of behavior who use non-PCT
control models study behavior using conventional IV-DV methodology.

Again, I suggest you clean your reading glasses. That was the second point of my post explicitly.

Students of behavior who use the PCT control model study behavior using the

test for the controlled variable (TCV).

Really? This is an interesting twist. How do you perform the Test on a computer program?

I would consider SD and PCT to be equivalent approaches to understanding

behavior if researchers who study behavior based on the SD model use the TCV > as their basic approach to understanding behavior.

They are not equivalent, not even in the same ballpark. The only similarity is that they both utilize negative feedback loops. SD is NOT a theory of behavior, nor is it a method devoted to control processes. You can model any number of things in SD.

The ‘rate’ equation in SD represents a policy or decision. It is was generates the behavior in an SD model, but that behavior can be from any source, human or not.

A final word from me on this. I am very disheartened, but I really shouldn’t be. Rick seems to be incapable of learning, and worse flaunts his ignorance as a badge of honor.

I would have asked in this post in any number of places where Rick saw me say what he took out of my post but I am not going to waste my time any longer with Rick.

I have no time for him and his nonsense. I will contact the folks who have shown an interest in my ideas privately.

Regards,

Marc

Regards,

Marc

[From Rick Marken (2005.12.07.1115)]

Marc Abrams (2005.12.07.1234) re: Rick Marken (2005.12.07.0840)

Just as might have a expected. Rick, you are a phoney and an ignorant one
with hubris to boot. An extremely lethal combination.

Gee, Marc. This seems like such a peculiar way to have a polite discussion.

System Dynamics is a modeling method. It is NOT a theory of human behavior.

Great. As a modeling approach I find it a bit cumbersome. I like to model
with the tools I have at hand and that are familiar. But if you want to use
SD then that's fine. What are we arguing about?

You do not know system dynamics nor will you ever, and that is a shame for
all concerned. You are so consumed keeping any knowledge out of your mind
that might infringe upon your loyalty to the PCT theory that I might be
better off talking to the Pope about converting to Judaism than getting you
to look at anything that touches on PCT.

If SD is just a modeling method then it can't possibly infringe on PCT. I've
even used it -- or something a lot like it -- Stella -- to make simple PCT
models. It has some nice properties, I'm just more comfortable with some
other ways of building models. I have nothing against SD as a modeling
method. It has it's good and bad features as a modeling method (from my
point of view) just like Java, C++, algebra and so on have their good and

No Rick, SD and PCT are NOT identical. SD utilizes negative feedback, and
also utilizes positive feedback. The various SD software packages like
Vensim, iThink, and Powersim offer many other options for mathematical
modeling as well.

Of course SD is not identical to PCT, any more than Pascal or Basic or
Stella is identical to PCT. But I think it's fair to say that these modeling
methods _are_ identical to PCT when they are used to model PCT, right?

What distinguishes PCT from other approaches to behavior based on control
theory (and the great insight and achievement of William T. Powers) is in
how PCT maps the control model to behavior...

What does any of this have to do with my posting?

Apparently nothing. I thought you were presenting SD as some kind of
alternative to PCT. But you're just presenting SD as a tool we could use to
develop our PCT models. I guess I'm just satisfied with the tools I'm using.

Students of behavior who use the PCT control model study behavior using the
test for the controlled variable (TCV).

Really? This is an interesting twist. How do you perform the Test on a
computer program?

We usually perform the test on people. We don't need to perform it on the
computer program because we know what variables the program is controlling.
We do the test on people to see if they are controlling the same variable
that the model of them -- the computer model -- is controlling. But I have
done the test on a computer program to show how the test is done. I
described such a test in my latest paper: the one published in JEP:HPP
(abstract at http://content.apa.org/journals/xhp/31/3). I certainly could
have done the modeling described in that paper using SD but it was just a
lot easier for me to do it using Visual Basic.

A final word from me on this. I am very disheartened, but I really shouldn't
be. Rick seems to be incapable of learning, and worse flaunts his ignorance

I'm sorry. The ignorance was accidental, not flaunted. I didn't understand
that you were just advocating SD a modeling methodology. I think SD sounds
like a great methodology. I'm just an old dog who likes to stick with the
few familiar tricks I know. So I'll probably stick with VisualBasic and
Java, though Bill and Bruce Abbott may eventually be able to lure me over to
Delphi.

Best

Rick

···

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Home: 310 474 0313
Cell: 310 729 1400

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This email message is for the sole use of the intended recipient(s) and
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[From Bryan Thalhammer (2005.12.07.1345 CST)]

Marc,

Your new leaf started out so promising, but I held my breath until now.

[Marc Abrams (2005.12.07.1234)]
Rick, you are a phoney and an ignorant one with hubris to boot. An extremely lethal combination.

For one who picks on professionalism, well, you have done it again.

...but I am not going to waste my time any longer with Rick.
I have no time for him and his nonsense.. I will contact the folks
who have shown an interest in my ideas privately.

Yes, Marc, please unsubscribe and stop running up the bill here but not paying.

--B.

···

Regards,
Marc

[Martin Taylor 2005.12.07.17.27]

[From Rick Marken (2005.12.07.0840)]

PCT is based on control theory. So the equations of PCT are exactly the same
as the equations of control theory. So if SD models are control models then
the SD equations will be identical to the PCT equations.

What distinguishes PCT from other approaches to behavior based on control
theory (and the great insight and achievement of William T. Powers) is in
how PCT maps the control model to behavior.

I'd argue that differently. So far as I can see, PCT is a specialization of SD, a specialization used to model human behaviour. It's a bit like object-oriented programming, where the object "John" may inherit all the properties and functions of the object "person", except those it explicitly overrides, plus other properties and functions specific to the onject "John."

SD allows any kind of interconnection among the different entities, with any choice of values. Testing an SD model involves making a specific set on interconnections and parameter values (the model) and then subjecting the model to different initial conditions (and perhaps external driving waveforms) to see what happens. Testing a PCT model is the same, except that usually "what happens" is compared to the observable behaviour of a person rather than that of a social or industrial system.

The specializations I see that distinguish PCT from general SD are three. One is that the universe under consideration has an explicit division into an "inside" and an "outside" (typically, but not always, that division corresponds to the human skin). The second is that most of the subsystems being analyzed are feedback loops that have parts that are inside, and parts that are outside (in "the environment"). General SD does not require feedback loops to exist, though they do exist in most "interesting" cases.

The third specialization is critically important.

In PCT, almost every feedback loop has two connections that provide signals from outside the loop, one from "inside" and one from the environment. Between each and the other, there is some transformation that can be interpreted as a "gain function". In general SD, even if the analysis produced a feedback loop of the PCT structure, the structure would carry no implication about the relationship between the two gain functions. In PCT, it is necessary that the gain from "inside" to "outside" be substantially greater than the gain from "outside" to "inside". This is the essential point that permits control. It ensures that the relative variation of what PCT calls "the perceptual signal" is less than that of what PCT calls the "output signal", and this is what allows the organism to deposit entroy into the environment. (Thermodynamically, it is absolutely critical for life).

Bottom line: PCT isn't distinct from SD. It's a special case with some required constraints and some conditions likely to occur in most cases. In that, it is like most specializations of SD to specific application areas. It just so happens that the application area of PCT is life, and within life, psychology. There may be other specializations of SD with the same application area, but they aren't PCT.

Martin

In a message dated 12/7/2005 2:44:54 P.M. Eastern Standard Time, bryanth@SOLTEC.NET writes:

···

[From Bryan Thalhammer (2005.12.07.1345 CST)]

Yes, Marc, please unsubscribe and stop running up the bill here but not
paying.

If you are having a problem I suggest you do the walking and this will be the last time I will be communicating with you for any reason.

I do not post for your pleasure or amusement. I post for mine, so go jump in the lake jerk-off

You obviously either have some major issues or you take too many drugs. In either case you are a waste of time for me.

From [Marc Abrams (2005.12.07.1822)]

In a message dated 12/7/2005 2:20:49 P.M. Eastern Standard Time, marken@MINDREADINGS.COM writes:

···

[From Rick Marken (2005.12.07.1115)]

Marc Abrams (2005.12.07.1234) re: Rick Marken (2005.12.07.0840)

Just as might have a expected. Rick, you are a phoney and an ignorant one
with hubris to boot. An extremely lethal combination.

Gee, Marc. This seems like such a peculiar way to have a polite discussion.

Is this post your way of apologizing for misreading my post and not caring enough about it to try to clarify before you replied? But the bigger issue here and the one that escapes you entirely is your lack of remorse for having made that mistake and the thinking that your reply played no part in why I said what I did above.

You you did not test your assumptions because you just knew you were ‘right’ and so far in every response to me I can say the same kind of thing in each post.

So either you see no connection to what you do and how I react or you simply don’t care. In either case I find it offensive.

Regards,

Marc

[From Rick Marken (2005.12.07.1550)]

Martin Taylor (2005.12.07.17.27) --

Rick Marken (2005.12.07.0840)

What distinguishes PCT from other approaches to behavior based on control
theory (and the great insight and achievement of William T. Powers) is in
how PCT maps the control model to behavior.

I'd argue that differently.

So would I, now that I know that SD is a modeling tool and not another model
of behavior based on control theory. What distinguishes PCT from SD is what
distinguishes quantum theory from calculus. One is a model that is designed
to explain observable phenomena (PCT, quantum theory) and the other is a
means of implementing that model quantitatively (SD, calculus).

So far as I can see, PCT is a specialization of SD

It looks to me like PCT is just one of many types of models that could be
implemented in SD. Saying that PCT is a specialization of SD is like saying
PCT is a specialization of FORTRAN.

Bottom line: PCT isn't distinct from SD. It's a special case with
some required constraints and some conditions likely to occur in most
cases.

I think this is like saying that PCT is a special case of algebra. That's
true, I suppose, but it seems a bit misleading to me (well, it mislead me,
anyway). SD is a modeling tool, like Delphi, VisualBasic, Java, Stella,
to the other tools you might use to implement a model and derive predictions
from it (which is why you use these tools in the first place). I think
whether or not you select SD for use as a modeling tool will depend on what
you want to get out of your modeling tool, not on some concept of how
similar PCT is to SD, or vice versa.

Best

Rick

···

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Home: 310 474 0313
Cell: 310 729 1400

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This email message is for the sole use of the intended recipient(s) and
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From [Marc Abrams (2005.12.07.1901)]

In a message dated 12/7/2005 6:55:51 P.M. Eastern Standard Time, marken@MINDREADINGS.COM writes:

···

[From Rick Marken (2005.12.07.1550)]

Martin Taylor (2005.12.07.17.27) –

It is nice to see that neither one of these guys read my post very well.

Rick and Martin, SD IS *** THE CALCULUS*** that is all it is. SD modeling tools have any number of other features added to them to enhance the modeling capability.

Like any other software tool these packages are intended to make modeling easier. You can still program a computer in hex but I doubt there are many who would want to do that especially in a Windows environment.

An SD package also forces you into proper structure according to the method. There are also optimization routines and validating methods that ordinary programing languages just don’t have.

PCT is NOT easily modeled in SD. But control is, and at my level of abstraction SD is the perfect tool.

The reason PCT is not easily modeled in SD is because although each utilizes ‘negative feedback loops’ as I tried showing yesterday, those feedback loops represent totally different levels of abstraction in each. A PCT control loop is NOT equivalent to an SD negative feedback loop.

Marc

[From Bryan Thalhammer (2005.12.1855 CST)]

Marc,

Marc, you know, this is not the comportment of someone who gives the impression of wanting to persuade a scientific community. Your behavior has again gone beyond the pale. It appears that what you do is start out being very nicey-picey, and then start to go way beyond your abilities, and then when, as in a scientific forum, people ask you questions, you lose it and get emotional at everybody. That is not the way to act here.

We ask you to make a sober proposal, with assertions, research questions, and experimental design. At first, you present a sketch of a model. But some people have difficulties with it, and ask you questions.

THEN YOU FLY OFF THE HANDLE, and start the name-calling again. You say, "Rick is a phoney and ignorant with hubris to boot." Ok. Hm. You also say, "Bryan has some major issues, and takes drugs." Ok. Fine. Well:

Marc Abrams cannot take criticism in a scientific forum where criticism is the ONLY way that knowledge grows. How is anyone EVER gonna take him seriously if he gets EMOTIONAL when someone asks reasonable questions?

Lakes around here are too frozen to jump in right now.

Cheers,

--Bryan

Marc Abrams wrote:

"I do not post for your pleasure or amusement. I post for mine, so go jump in the lake jerk-off.

You obviously either have some major issues or you take too many drugs. In either case you are a waste of time for me."

[Martin Taylor 2005.12.07.22.49]

[From Rick Marken (2005.12.07.1550)]

Martin Taylor (2005.12.07.17.27) --

Rick Marken (2005.12.07.0840)

What distinguishes PCT from other approaches to behavior based on control
theory (and the great insight and achievement of William T. Powers) is in
how PCT maps the control model to behavior.

I'd argue that differently.

So would I, now that I know that SD is a modeling tool and not another model
of behavior based on control theory.

No, it's a field of study, that uses tools for modelling the same way PCT does.

What distinguishes PCT from SD is what
distinguishes quantum theory from calculus. One is a model that is designed
to explain observable phenomena (PCT, quantum theory) and the other is a
means of implementing that model quantitatively (SD, calculus).

I think you msiunderstand. System Dynamics is the study of the dynamics of systems. It most definitely IS applied in constructing models that are designed to explain observable phenomena, and moreover, to predict them. Any kind of system that involves materials or information (i.e. signals) passing from one place to another, and perhaps being stored in some of the places. The structure of a pure HPCT network is such a system. So is the structure of any of the many variants on pure HPCT that have been proposed over the years.

System Dynamics is not a calculus. System Dynamicists do have techniques for calculation, and there are lots of software packages that help one to model this or that system structure to see the effects of parametric variation in matching models to observable data.

> So far as I can see, PCT is a specialization of SD

It looks to me like PCT is just one of many types of models that could be
implemented in SD. Saying that PCT is a specialization of SD is like saying
PCT is a specialization of FORTRAN.

That makes no sense at all. I suppose you might say that a PCT simulation program written in Fortran is a specialization of Fortran. But even that would be wrong. Such a programme would be an application of Fortran. PCT is not an application of System Dynamics. It's a subset, a specialization.

Bottom line: PCT isn't distinct from SD. It's a special case with
some required constraints and some conditions likely to occur in most
cases.

I think this is like saying that PCT is a special case of algebra.

Likewise, this makes no sense. System Dynamics is the study of dynamic systems in general. PCT is the study of dynamic systems that have certain specific characteristics.

Whatever tools are valid for studying system dynamics in general are also valid for studying PCT, but, being general, they probably won't be as useful as tools specialized for studying PCT.

Martin

From [Marc Abrams (2005.12.07.2327)]

In a message dated 12/7/2005 11:04:23 P.M. Eastern Standard Time, mmt-csg@ROGERS.COM writes:

POS Chap - 1.pdf (456 KB)

POS Chap 10.pdf (724 KB)

···

[Martin Taylor 2005.12.07.22.49]

Martin I am afraid you are not quite correct.

First, it is not a field of study. It is used to study various fields

Second, if by ‘dynamic’ you mean through time then you are correct, but if you meant ‘chaotic’ you are mistaken. I took the liberty of attaching Chapters 1 and 10 from Principles of Systems for your review

Chapter one provides the background and reasoning behind SD and Chapter 10 on the mathematics involved.

SD is not a calculus, it utilizes Integration (calculus) as it’s foundation and it is intended to be used to study continuous feedback systems.

Regards,

Marc

First of all from Principles of Systems, 1960 Jay Forrester

[From Rick Marken (2005.12.07.1550)]

Martin Taylor (2005.12.07.17.27) –

Rick Marken (2005.12.07.0840)

What distinguishes PCT from other approaches to behavior based on control
theory (and the great insight and achievement of William T. Powers) is in
how PCT maps the control model to behavior.

I’d argue that differently.

So would I, now that I know that SD is a modeling tool and not another model
of behavior based on control theory.

No, it’s a field of study, that uses tools for modelling the same way PCT does.

What distinguishes PCT from SD is what
distinguishes quantum theory from calculus. One is a model that is designed
to explain observable phenomena (PCT, quantum theory) and the other is a
means of implementing that model quantitatively (SD, calculus).

I think you msiunderstand. System Dynamics is the study of the
dynamics of systems. It most definitely IS applied in constructing
models that are designed to explain observable phenomena, and
moreover, to predict them. Any kind of system that involves materials
or information (i.e. signals) passing from one place to another, and
perhaps being stored in some of the places. The structure of a pure
HPCT network is such a system. So is the structure of any of the many
variants on pure HPCT that have been proposed over the years.

System Dynamics is not a calculus. System Dynamicists do have
techniques for calculation, and there are lots of software packages
that help one to model this or that system structure to see the
effects of parametric variation in matching models to observable data.

So far as I can see, PCT is a specialization of SD

It looks to me like PCT is just one of many types of models that could be
implemented in SD. Saying that PCT is a specialization of SD is like saying
PCT is a specialization of FORTRAN.

That makes no sense at all. I suppose you might say that a PCT
simulation program written in Fortran is a specialization of Fortran.
But even that would be wrong. Such a programme would be an
application of Fortran. PCT is not an application of System Dynamics.
It’s a subset, a specialization.

Bottom line: PCT isn’t distinct from SD. It’s a special case with
some required constraints and some conditions likely to occur in most
cases.

I think this is like saying that PCT is a special case of algebra.

Likewise, this makes no sense. System Dynamics is the study of
dynamic systems in general. PCT is the study of dynamic systems that
have certain specific characteristics.

Whatever tools are valid for studying system dynamics in general are
also valid for studying PCT, but, being general, they probably won’t
be as useful as tools specialized for studying PCT.

Martin

[From Rick Marken (2005.12.07.2250)]

Martin Taylor (2005.12.07.22.49)--

Rick Marken (2005.12.07.1550)

So would I, now that I know that SD is a modeling tool and not another model
of behavior based on control theory.

No, it's a field of study, that uses tools for modelling the same way PCT does.

I'm so confused! Marc said it's a modeling tool; you say it's a field of study. I feel like Jack Nicholson (Jake Gittes) dealing with Faye Dunaway (Evelyn Mulwray) in _Chinatown_:
Evelyn Mulwray: SD is a field of study.
[Gittes slaps Evelyn]
Jake Gittes: I said I want the truth!
Evelyn Mulwray: SD is a modeling tool...
[slap]
Evelyn Mulwray: SD is a field of study...
[slap]
Evelyn Mulwray: Field of study, modeling tool.
[More slaps]
Jake Gittes: I said I want the truth!
Evelyn Mulwray: It's a field of study AND a modeling tool!

I think you msiunderstand. System Dynamics is the study of the dynamics of systems. It most definitely IS applied in constructing models that are designed to explain observable phenomena, and moreover, to predict them.

Could you give me an example of the application of Systems Dynamics in constructing, say, a simple control model?

Wait, I think I heard someone say "Forget it, Jake, it's Chinatown".

Best

Rick

···

---
Richard S. Marken
Home 310 474-0313
Cell 310 729-1400

[Martin Taylor 2005.12.08.11.11]

[From Rick Marken (2005.12.07.2250)]

Martin Taylor (2005.12.07.22.49)--

Rick Marken (2005.12.07.1550)

So would I, now that I know that SD is a modeling tool and not another model
of behavior based on control theory.

No, it's a field of study, that uses tools for modelling the same way PCT does.

I'm so confused! Marc said it's a modeling tool; you say it's a field of study.

Could you give me an example of the application of Systems Dynamics in constructing, say, a simple control model?

Well, from my point of view, every PCT model is the kind of example you want.

I think that what Marc may be talking about is a set of conventional tools normally discussed on the System Dynamics mailing list, which I read and very occasionally contribute to (and from where I was originally pointed toward PCT). Those tools are used to study the dynamics of systems in other application areas, where the asymmetry of gain between two halves of a feedback loop that has two external inputs is not an issue. In those applications, the very concept of "gain" is a little dubious, since quite often one is talking about the transmission of conserved material from one "stock" to another by means of "flows".

If you want to do that kind of analysis of a PCT control loop, you do have a conserved quantity, which is energy. In PCT analysis, energy is usually forgotten. It is, however, critical. The effectiveness of the PCT loop for the maintenance of life depends on minimizing the energy that comes in from outside in a potentially disruptive way, and maximizing the potential use of energy to avoid that disruption. That outflow of energy has to derive from somewhere. Potentially disruptive energy input comes in many forms and scales, which is why there must be PCT-type control loops at many scales, from molecular to philosophical.

From the "stock and flow" kind of approach, the PCT loop probably could be seen as using small energy flows to control valves that obstruct or release large energy flows. After all, that is physically what happens (through many transofmative stages). But that kind of approach is only one approach to system dynamics, which is, after all, the study of the dynamics of systems.

Martin

From [Marc Abrams (2005.12.08.1311)]

In a message dated 12/8/2005 1:51:42 A.M. Eastern Standard Time, marken@MINDREADINGS.COM writes:

···

[From Rick Marken (2005.12.07.2250)]

Are you still confused? Did my attached Chapters of Principles of Systems help explain?

Regards,

Marc

From [Marc Abrams (2005.12.08.1320)]

In a message dated 12/8/2005 11:34:27 A.M. Eastern Standard Time, mmt-csg@ROGERS.COM writes:

···

[Martin Taylor 2005.12.08.11.11]

Another wonderful example of advocacy without inquiry and an attempt to remain in unilateral control of the environment.

My original post on the subject of System Dynamics concerned the work of Jay Forrester and the modeling paradigm he developed. I even used pages out of one of his books as examples of what I was trying to say.

So what led to Rick’s ‘confusion’ was that Martin Taylor did not post on my topic, he posted on what*** he*** wanted to advocate about, which is about what he wants to discuss.

It might have been helpful for all concerned if Martin had told everyone that in fact system dynamics and the study of dynamical systems are not
equivalent.

Just like the use of SD and PCT are not.

Dynamical systems are indeed a field of study and most people know that field as the study of ‘chaos’.

Martin, do you actually read my posts? It seems the only reason you responded to my diagram was because you thought you did something similar a number of years ago, but as I posted to you privately, although our diagrams are similar we are looking at different levels of abstraction so in fact our diagrams do not represent the same set of ideas.

I might be hallucinating but your interest in my diagram took a nose dive after that post. Hmm, I wonder why? Can you help me out here? I say this because have not responded to any of my posts since; privately or here on CSGnet.

Advocacy without inquiry makes for poor dialogue, but like Rick, I don’t think you were, or are, interested in any dialogue. My hypothesis is that you are looking to maintain your ‘status’, ‘win’ and stay in control of your environment. What are your thoughts on the matter?

Regards,

Marc

[From Rick Marken (2005.12.07.1120)]

Martin Taylor (2005.12.08.11.11)--

Rick Marken (2005.12.07.2250)]

Could you give me an example of the application of Systems Dynamics
in constructing, say, a simple control model?

Well, from my point of view, every PCT model is the kind of example you want.

I've been building PCT models without any knowledge of Systems Dynamics
(whatever that is -- tools or point of view) and I don't feel like I've
missed anything. Just plain old mathematics works for me.

I think that what Marc may be talking about is a set of conventional
tools normally discussed on the System Dynamics mailing list

Yes, but those are just like subroutines or objects that do things like
accumulation (stocks) without you having to write all the code to do the
accumulation. Isn't that right? If so, then Systems Dynamics is a toolkit
(not a point of view), much like a Fortran function library. In Systems
Dynamics the functions are things like stock and flow instead of square root
and cosine? Is that it? If so, that's very nice but in my modeling the stuff
that is tough to do is modeling the perceptual functions. In order to
develop a tool box of perceptual functions you'd first have to do the
research to see what those perceptual functions are. And that's basically
what the PCT research program is about; finding out the different kinds of
perceptual functions that exist in organisms (there will be a lot) and how
they are organized in groups of control systems.

Best

Rick

···

---
Richard S. Marken
Home: 310 474 0313
Cell: 310 729 1400

--------------------

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Re: System Dynamics
[Martin Taylor 2005.12.08.14.05]

From
[Marc Abrams (2005.12.08.1320)]

In a
message dated 12/8/2005 11:34:27 A.M. Eastern Standard Time,
mmt-csg@ROGERS.COM writes:

[Martin Taylor
2005.12.08.11.11]

Dynamical systems are
indeed a field of study and most people know that field as the study
of ‘chaos’.

You couldn’t be more wrong! Sorry, but the study of chaos is a
small but popular subset of SD. (Aside: For my part, I find systems
that re not chaotic but are on “the edge of chaos” much more
interesting, since they seem to show up in all sorts of evolved – as
opposed to constructed – dynamical systems; in particular, they
almost invariably show up in systems that involve living
things.)

I might be
hallucinating but your interest in my diagram took a nose dive after
that post. Hmm, I wonder why?

I was afraid of your going into one of your abusive phases – a
fear that you have more than adequately proven to be justified.

Furthermore, I really don’t have any
interest in metaphor except as an interesting object of psychological
study. Once you aserted for a scond time that your picture represented
only an airy metaphor, there was nothing more to be said about it
until you made the metaphor into something serious.

My original post on the subject
of System Dynamics concerned the work of Jay Forrester and the
modeling paradigm he developed. I even used pages out of one of his
books as examples of what I was trying to say.

I read those (they were chapters 1 and 10 of a typescript).
Chapter 1 seemed to be a good illustration of how PCT is indeed a
specialization of SD (assuming the book to be about SD, as the
introduction claims). Without using the term “Perceptual Control”
it does show the PCT basic control loop (minus the disturbance). It
could serve as a quite reasonable introduction to a PCT text. Chapter
10 is about high-school calculus and its application to feedback
loops. What was a reader supposed to glean from this that s/he would
not have known on high-school graduation?

So what led to Rick’s
‘confusion’ was that Martin Taylor did not post on my topic, he posted
is about what * he* wants to
discuss.

Don’t we all? You assert explicitly in a high perceptage of your
posts that you aren’t interested in discussing what other people want
to discuss. Should you be surprised if other poeple don’t necessarily
want to discuss what you do? I don’t think it’s a criticism of you
that you have your own interests, but it is a criticism of you that
you demand that everyone else share those interests to the exclusion
of all other interests.

It might have been
helpful for all concerned if Martin had told everyone that in fact
system dynamics and the study of dynamical systems arenot equivalent.

I wouldn’t have told people that untruth.

I might have said that many of the people on the System Dynamics
list study particular kinds of dynamical systems, usually involving
social interaction, but often natural dynamic systems that don’t
involve humans. But it would have been irrelevant.

Sorry, folks. I realize that I violated my own rule by replaying
to a posting by Marc, but he had been quite reasonable up to that
point, and I had mistakenly perceived that he had turned over a new
leaf and was willing to contribute to the scientific discussion. I’ve
double violated it by sending this message, but I’ll try to stick to
my rule in future.

Martin

[Martin taylor 2005.12.08.14.36]

[From Rick Marken (2005.12.07.1120)]

Martin Taylor (2005.12.08.11.11)--

Rick Marken (2005.12.07.2250)]

Could you give me an example of the application of Systems Dynamics
in constructing, say, a simple control model?

Well, from my point of view, every PCT model is the kind of example you want.

I've been building PCT models without any knowledge of Systems Dynamics
(whatever that is -- tools or point of view) and I don't feel like I've
missed anything. Just plain old mathematics works for me.

I'm not going to suggest you have missed anything. Personally, I find it often helpful (or at least interesting) to know something of the larger subject of which the thing that primarily interests me is an example. Not everyone feels this way, and there's no harm in that. Do what you do well.

> I think that what Marc may be talking about is a set of conventional

tools normally discussed on the System Dynamics mailing list

Yes, but those are just like subroutines or objects that do things like
accumulation (stocks) without you having to write all the code to do the
accumulation. Isn't that right?

That's more or less correct.

If so, then Systems Dynamics is a toolkit

No. The toolkit of System Dynamics is a toolkit.

(not a point of view),

Not a point of view, but a field of study.

much like a Fortran function library. In Systems
Dynamics the functions are things like stock and flow instead of square root
and cosine?

No. square root and sine and cosine, and all the apparatus of calculus are part of the toolkit. As Marc keeps saying, the mathematical background is the same. To make PCT out of system dynamics merely involves asserting some special constraints about the structure and the parameters of the system under study (Again, I redundantly emphasise the asymmetry between input gain and output gain, which is the crucial aspect of effective control).

Is that it? If so, that's very nice but in my modeling the stuff
that is tough to do is modeling the perceptual functions.

Agreed. For the visual periphery, all the work that followed on Hubel and Weisel provides clues, but when you get into more abstract levels, there aren't any physiological clues (at least none that I know of, beyond the gross NMR and similar stuff).

In order to
develop a tool box of perceptual functions you'd first have to do the
research to see what those perceptual functions are. And that's basically
what the PCT research program is about; finding out the different kinds of
perceptual functions that exist in organisms (there will be a lot) and how
they are organized in groups of control systems.

That's a reasonable way of looking at PART of the PCT research program. But even a single canonical control loop contains more than its perceptual input function. An equally difficult and relevant area of research would be on the Reference Input Functions. Tolerance and nonlinearity, the precise roles of memory and constructive imagination, are a couple of others. Another would be to address the mechanisms whereby the choices of which perceptions are controlled at any moment vary over time. There are lots more. To concentrate on elucidating the perceptual functions is necessary, but it's not sufficient.

Back to System Dynamics. Inasmuch as most of the application areas addressed by the more general area do not involve the asymmetrical gain that characterizes control, SD in its more general sense might be a suitable approach to studying the ways systems of control systems interact. I don't know, but there might be general theorems or guiding principles that would apply, and could guide more specific research.

It's a thought, perhaps an idle one.

Martin