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gjm comments on What is control theory, and why do you need to know about it? - Less Wrong
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Certainly I, as the designer, had a model of the robot and its environment when I wrote that program, and the program implements those models. But the robot itself has no model of its environment. It calculates the positions of its feet, relative to itself, by sensing its joint angles, knowing the lengths of its limb segments and calculating, so it does have a fairly limited model of itself: it knows its own dimensions. However, it does not know its own mass, or the characteristics of its sensors and actuators.
The fact that it works does not mean that it has an "implicit" model of the environment: "implicit", in a context like this, means "not". What is a model? A model is a piece of mathematics in which certain quantities correspond to certain properties of the thing modelled, and certain mathematical relationships between these correspond to certain physical relationships. Maxwell's equations model electromagnetic phenomena. The Newton-Raphson (EDIT: I meant Navier-Stokes) equation models fluid flow. "Implicit model" is what one says, when one expects to find a model and finds none. The robot's environment contains a simulated wind pushing on the robot, and a simulated hand giving it a shove. The robot knows nothing of this: there is no variable in the part of the program that deals with the robot's sensors, actuators, and control algorithms that represents the forces acting on it. The robot no more models its environment than a thermostat models the room outside it.
Since it is possible to build systems that achieve goals without models, and also possible, but in general rather more complicated, to build such systems that do use models, I do not think that the blind god of evolution is likely to have put models anywhere. It has come up with something -- us, and probably the higher animals -- that can make models, but nothing currently persuades me that models are how brains must work. I see no need of that hypothesis.
I'd rather like to build that robot. If I did, I would very likely use an onboard computer just to have flexibility in reconfiguring its control algorithms, but the controllers themselves are just PID loops. If, having got it to work robustly, I were to hard-wire it, the control circuitry would consist of a handful of analogue components for each joint, and no computer required. I still find it remarkable, how much it can do with so little.
Er, I think you mean Navier-Stokes.
I think that's unfair. The notion of an implicit model (meaning something like "a model such that a system making use of it would behave just like this one") is a useful one; for instance, suppose you are presented with a system designed by someone else that isn't working as it should; one way to diagnose its troubles is to work out what assumptions about the world are implicit in its design (they might not amount to anything quite so grand as a "model", I suppose) and how they fail to match reality, and then -- with the help of one's own better model of the world -- to adjust the system's behaviour.
Or, of course, you can just poke at it until it behaves better. But then I'd be inclined to say that you're still using a model of the world -- you're exploiting the world's ability to be used as a model of itself. If a system gets "poked at until it behaves better" often enough and in varied enough ways, it can end up with a whole lot of information about the world built into it. If you don't want to call that an "implicit model", fair enough; but what's wrong with doing so?
Poking at it until it works isn't revising a model, in the same sense that walking toward the pole star when you want to go North isn't cartography.
I didn't say that poking at something until it works is revising a model, I said that it's using a model (in, doubtless, a rather trivial sense). And, if I'm understanding your analogy right, surely the analogous claim would be that walking (as nearly as possible given that one remains on the surface of the earth) towards the pole star isn't reading a map (even an "implicit" one), not that it isn't cartography; and I don't think that's quite so obvious. (Also: it seems to me that "maps" have more in common than "models", and I think that's relevant.)