[amatic]

That's interesting, I've never heard of Milton Friedman's thermostat, but there is a procedure called "The test for the controlled variable" intended to find if a system is controlling one variable or another, invented by W.T. Powers sometime in the fifties, I think. You make a hypothesis about what is the controlled variable, then you apply a disturbance. If the variable stays relatively undisturbed, the correlation between the disturbance and the controlled variable will be nearly zero.