[amatic]

There is a mistake right in the beginning, not sure how it affects the conclusions yet. The variables given are S - System variable (some kind of disturbance), Z is the outcome ( a controlled variable) and R is the action of a controller. The causal relations between them are S affects Z, S affects R, and R affects Z.

> The archetypal example for this is something like a thermostat. The variable S represents random external temperature fluctuations. The regulator R is the thermostat, which measures these fluctuations and takes an action (such as putting on heating or air conditioning) based on the information it takes in. The outcome Z is the resulting temperature of the room, which depends both on the action taken by the regulator, and the external temperature.

The problem here is that the regulator R does not measure external temperature. It just measures the controlled variable - the temperature Z, so the causal arrow should go from Z to R too, and the arrow from S to R does not exist.

[masfuerte]

> The problem here is that the regulator R does not measure external temperature.

Domestic thermostats typically don't but some heating control systems do.

[analog31]

I wonder if the theorem is another way of showing how hard control is without feedback. And I can't quite figure out if it addresses dynamic systems as opposed to static ones.