7 ms·

Every good regulator of a system must be a model of that system (1970) [pdf]

92 points by ismail 7y ago

crawfordcomeaux 7y ago
If we wish to be good regulators of the environment, we must be a model of the environment. What does this mean?
- sleavey 7y ago
  That if we wish to control some figure of merit, such as the average global temperature or sea level, then we must understand which variables influence them.
- nabla9 7y ago
  You must figure out the causal structure of the system you are trying to regulate and incorporate it into the regulator. Using some generic and simple model to observe variables will not do. When the system becomes very complex this involves simulation. Think the game of chess as a system you try to regulate towards win. The only working solution is to simulate gameplay, dong sequences of moves and evaluating their outcomes (search). Chess computer replicates the system it regulates.
sytelus 7y ago
This is very interesting in more modern fields of reinforcement learning where debates rages between "model-free" and "model-based" approaches. I'd no idea that there was actual any proofs around this. However, I have to say the theorem goes against my intuition. If you want to control a heating system, you don't really need to build the model of heater which could be very complex requiring the knowledge of how it works internally. Instead, you observe the temperature for given action and successfully control it without ever knowing how your heater actually works. You do build a model but its model of observations and actions, not the heater itself. In fact, for vast majority of things we control such as bicycle or fan or oven, people rarely knows the model of these very complex systems (in terms physical principals and internals).
- sleavey 7y ago
  I guess you could make it more intuitive by thinking about the system in terms of the units of your feedback (e.g. force on an actuator like a bicycle handle bar in your example) rather than the deeper, underlying physics. In the end it doesn't matter which you use as long as it works.
- BoiledCabbage 7y ago
  > You do build a model but its model of observations and actions, not the heater itself. In fact, for vast majority of things we control such as bicycle or fan or oven, people rarely knows the model of these very complex systems (in terms physical principals and internals). I think you're mistaking the term model for a replica. You don't need a replica, but you do need a model for your relevant tasks. If the heater needs 30 mins to warm up before it starts pumping heat, you'll need to model that. If it overheats after running for 3hrs straight you'll need to model that too. You don't need every molecule of the physical item replicated but you do need to model all of the relevant behaviors.
  - dTal 7y ago
    This sounds a little circular. If a "model" is defined as a representation of a system of sufficient fidelity to control it, then of course all controllers contain models. Deciding which behaviors are "relevant" would appear to be the key issue here.
    - mannykannot 7y ago
      You have a point, but I think it is useful to have a unifying general principle to use in analyzing the design of regulators, and having a theoretical basis makes it harder to dismiss. I can think of a couple of places where it could have avoided accidents (one tragic and one expensive.) The 1974 DC10 crash in Paris was supposed to be impossible because the airplane could not be pressurized unless the cargo door was properly latched, but the mechanism depended on the position of the handle rather than the latching pins. At Three Mile Island, the operators were trained to use the pressurizer water level as a primary indicator of the state of the system, and turned off the emergency cooling feed as a consequence.
_glass 7y ago
In the management sciences this is one argument for diversity. If you have a complex environment, then your team has to mirror that complexity. But the whole thing based on system theory, so the basic assumption is that there is no direct access to the internal parts of the system, that is difficult if one assume that they are human actors.
- 9q9 7y ago
  Why would "diversity" help modelling the complexity of the environment? Would a chess-playing program get better if it also contained a go-engine, checker-engine, scissor-paper-rocks rules and tiddly-winks?
  - TeMPOraL 7y ago
    I think what they mean is that your market is the system, and your team is the controller, so if the market is diverse, your team should be diverse too. I think it's still cargo-cult control theory. You can't just make your team as diverse as your market and expect results to magically improve.
    - 9q9 7y ago
      I agree that that's what they mean. What I'm not seeing is how the "requisite variety" that control theory talks about, relates to "diversity" in modern usage (which for practical purposes is coeval with the political demand of hiring fewer members of certain demographics). Let's use a less loaded analogy, intellectual diversity: the Manhattan Project, which needed to adapt to a most complex and adversarial environment (world war), hired a lot of STEM scientists, in particular mathematicians, physicists and chemists, and mostly from a few top universities, in other words, a highly homogeneous group that were educated on the same few core subjects (linear algebra, real and complex anaysis, Newtonian and quantum physics, special and general relativity). I do not believe that the Manhattan Project would have been better by hiring fewer STEM graduates, and more psychologists, historians, yoga teachers and so forth. How would modern management science convince me that I'm wrong? The heart of the issue is the conflation of "diversity" (a political concept) with control theory's "requisite variety". The two are related, but not the same. I partly agree that it's cargo-culting, but that's not the whole story: the Melanesians after whom Cargo Cult is modelled, would probably not have tried to fire / doxx / cancel anyone who wasn't on board with the belief that building of an airplane runway will by itself bring desirable Western goods.
longtom 7y ago
Previous discussion: https://news.ycombinator.com/item?id=16545537 https://news.ycombinator.com/item?id=16545537
ur-whale 7y ago
It's funny, because I initially thought the title was about regulators as in "government regulators", not about control theory. Even more interesting is the fact that this idea from control theory in fact completely transposes to government regulation, as in: it'd be really good if regulators had the faintest idea what they're doing (aka in a perfect world, they would have a somewhat decent model of the system they're trying to regulate). Except ... that's very probably not the case ... (I'm looking at you economists).
- jplayer01 7y ago
  > Except ... that's very probably not the case ... (I'm looking at you economists). I'm not sure I agree. From what I've seen, it's usually politicians that ignore unintended consequences and incentives that lead to poor outcomes. Not sure what that has to do with economists ... they don't run the country.
carrolldunham 7y ago
This paper is so popular all over the web but nobody has ever managed to follow the proof of its 'theorem'. This guy is a math professor and quantum physicist and he was unable to extract any sense from it: https://johncarlosbaez.wordpress.com/2016/01/27/the-good-regulator-theorem/ https://johncarlosbaez.wordpress.com/2016/01/27/the-good-reg... He points out that the proof uses terms that aren't even defined. The fact that the authors of the paper are a psychiatrist and, I think a humanities academic, doesn't fill me with confidence that the 'theorem' and its 'proof' are actually those things. It's disquieting to see so many people taking the paper's claim and running with it because it 'resonates'. I know there is also an 'internal model principle' paper which is technically correct but that applies to a specific set of linear systems under specific conditions.
- tlb 7y ago
  I felt like I followed the proof. The claim is actually very narrow: that to be a perfect regulator of a system you need an exact model. Obviously, lots of devices in our world can regulate things reasonably well with simple models. For instance, a standard thermostat keeps your house at 20 +/- 2 degrees C. In order to keep it at 20.00000000 exactly, it might need to model every air molecule bouncing around.
  - marcosdumay 7y ago
    That's a problem, because we don't actually use "perfect" regulators for any system. I'm very willing to believe those things don't even exist. So, it may be a correct proof, but it's a useless one.
    - staticautomatic 7y ago
      The implication being what, exactly?
      - marcosdumay 7y ago
        Well, except for the fact that the article is trivially wrong, and that there is no reason to trust its conclusions, I don't think there are any. But you didn't need my comment for that, the fact that there is an entire engineering discipline thriving on doing exactly what the article claims to be impossible should suffice.
carapace 7y ago
One of the main results of Cybernetics. You can get a PDF of Ashby's book "Introduction to Cybernetics" from here, as well as a lot of other information: http://pespmc1.vub.ac.be/ASHBBOOK.html\ http://pespmc1.vub.ac.be/ASHBBOOK.html\ (edit: Ha! I just realized it's the same Principia Cybernetica Web!)

92 points by ismail 7y ago

crawfordcomeaux 7y ago

If we wish to be good regulators of the environment, we must be a model of the environment. What does this mean?

sleavey 7y ago
That if we wish to control some figure of merit, such as the average global temperature or sea level, then we must understand which variables influence them.
nabla9 7y ago
You must figure out the causal structure of the system you are trying to regulate and incorporate it into the regulator. Using some generic and simple model to observe variables will not do. When the system becomes very complex this involves simulation. Think the game of chess as a system you try to regulate towards win. The only working solution is to simulate gameplay, dong sequences of moves and evaluating their outcomes (search). Chess computer replicates the system it regulates.

sytelus 7y ago

This is very interesting in more modern fields of reinforcement learning where debates rages between "model-free" and "model-based" approaches. I'd no idea that there was actual any proofs around this. However, I have to say the theorem goes against my intuition. If you want to control a heating system, you don't really need to build the model of heater which could be very complex requiring the knowledge of how it works internally. Instead, you observe the temperature for given action and successfully control it without ever knowing how your heater actually works. You do build a model but its model of observations and actions, not the heater itself. In fact, for vast majority of things we control such as bicycle or fan or oven, people rarely knows the model of these very complex systems (in terms physical principals and internals).

sleavey 7y ago
I guess you could make it more intuitive by thinking about the system in terms of the units of your feedback (e.g. force on an actuator like a bicycle handle bar in your example) rather than the deeper, underlying physics. In the end it doesn't matter which you use as long as it works.
BoiledCabbage 7y ago
> You do build a model but its model of observations and actions, not the heater itself. In fact, for vast majority of things we control such as bicycle or fan or oven, people rarely knows the model of these very complex systems (in terms physical principals and internals). I think you're mistaking the term model for a replica. You don't need a replica, but you do need a model for your relevant tasks. If the heater needs 30 mins to warm up before it starts pumping heat, you'll need to model that. If it overheats after running for 3hrs straight you'll need to model that too. You don't need every molecule of the physical item replicated but you do need to model all of the relevant behaviors.
- dTal 7y ago
  This sounds a little circular. If a "model" is defined as a representation of a system of sufficient fidelity to control it, then of course all controllers contain models. Deciding which behaviors are "relevant" would appear to be the key issue here.
  - mannykannot 7y ago
    You have a point, but I think it is useful to have a unifying general principle to use in analyzing the design of regulators, and having a theoretical basis makes it harder to dismiss. I can think of a couple of places where it could have avoided accidents (one tragic and one expensive.) The 1974 DC10 crash in Paris was supposed to be impossible because the airplane could not be pressurized unless the cargo door was properly latched, but the mechanism depended on the position of the handle rather than the latching pins. At Three Mile Island, the operators were trained to use the pressurizer water level as a primary indicator of the state of the system, and turned off the emergency cooling feed as a consequence.

_glass 7y ago

In the management sciences this is one argument for diversity. If you have a complex environment, then your team has to mirror that complexity. But the whole thing based on system theory, so the basic assumption is that there is no direct access to the internal parts of the system, that is difficult if one assume that they are human actors.

9q9 7y ago
Why would "diversity" help modelling the complexity of the environment? Would a chess-playing program get better if it also contained a go-engine, checker-engine, scissor-paper-rocks rules and tiddly-winks?
- TeMPOraL 7y ago
  I think what they mean is that your market is the system, and your team is the controller, so if the market is diverse, your team should be diverse too. I think it's still cargo-cult control theory. You can't just make your team as diverse as your market and expect results to magically improve.
  - 9q9 7y ago
    I agree that that's what they mean. What I'm not seeing is how the "requisite variety" that control theory talks about, relates to "diversity" in modern usage (which for practical purposes is coeval with the political demand of hiring fewer members of certain demographics). Let's use a less loaded analogy, intellectual diversity: the Manhattan Project, which needed to adapt to a most complex and adversarial environment (world war), hired a lot of STEM scientists, in particular mathematicians, physicists and chemists, and mostly from a few top universities, in other words, a highly homogeneous group that were educated on the same few core subjects (linear algebra, real and complex anaysis, Newtonian and quantum physics, special and general relativity). I do not believe that the Manhattan Project would have been better by hiring fewer STEM graduates, and more psychologists, historians, yoga teachers and so forth. How would modern management science convince me that I'm wrong? The heart of the issue is the conflation of "diversity" (a political concept) with control theory's "requisite variety". The two are related, but not the same. I partly agree that it's cargo-culting, but that's not the whole story: the Melanesians after whom Cargo Cult is modelled, would probably not have tried to fire / doxx / cancel anyone who wasn't on board with the belief that building of an airplane runway will by itself bring desirable Western goods.

longtom 7y ago

Previous discussion: https://news.ycombinator.com/item?id=16545537 https://news.ycombinator.com/item?id=16545537

ur-whale 7y ago

It's funny, because I initially thought the title was about regulators as in "government regulators", not about control theory. Even more interesting is the fact that this idea from control theory in fact completely transposes to government regulation, as in: it'd be really good if regulators had the faintest idea what they're doing (aka in a perfect world, they would have a somewhat decent model of the system they're trying to regulate). Except ... that's very probably not the case ... (I'm looking at you economists).

jplayer01 7y ago
> Except ... that's very probably not the case ... (I'm looking at you economists). I'm not sure I agree. From what I've seen, it's usually politicians that ignore unintended consequences and incentives that lead to poor outcomes. Not sure what that has to do with economists ... they don't run the country.

carrolldunham 7y ago

This paper is so popular all over the web but nobody has ever managed to follow the proof of its 'theorem'. This guy is a math professor and quantum physicist and he was unable to extract any sense from it: https://johncarlosbaez.wordpress.com/2016/01/27/the-good-regulator-theorem/ https://johncarlosbaez.wordpress.com/2016/01/27/the-good-reg... He points out that the proof uses terms that aren't even defined. The fact that the authors of the paper are a psychiatrist and, I think a humanities academic, doesn't fill me with confidence that the 'theorem' and its 'proof' are actually those things. It's disquieting to see so many people taking the paper's claim and running with it because it 'resonates'. I know there is also an 'internal model principle' paper which is technically correct but that applies to a specific set of linear systems under specific conditions.

tlb 7y ago
I felt like I followed the proof. The claim is actually very narrow: that to be a perfect regulator of a system you need an exact model. Obviously, lots of devices in our world can regulate things reasonably well with simple models. For instance, a standard thermostat keeps your house at 20 +/- 2 degrees C. In order to keep it at 20.00000000 exactly, it might need to model every air molecule bouncing around.
- marcosdumay 7y ago
  That's a problem, because we don't actually use "perfect" regulators for any system. I'm very willing to believe those things don't even exist. So, it may be a correct proof, but it's a useless one.
  - staticautomatic 7y ago
    The implication being what, exactly?
    - marcosdumay 7y ago
      Well, except for the fact that the article is trivially wrong, and that there is no reason to trust its conclusions, I don't think there are any. But you didn't need my comment for that, the fact that there is an entire engineering discipline thriving on doing exactly what the article claims to be impossible should suffice.

carapace 7y ago

One of the main results of Cybernetics. You can get a PDF of Ashby's book "Introduction to Cybernetics" from here, as well as a lot of other information: http://pespmc1.vub.ac.be/ASHBBOOK.html\ http://pespmc1.vub.ac.be/ASHBBOOK.html\ (edit: Ha! I just realized it's the same Principia Cybernetica Web!)