> control purposes Here you have an implicit assumption that the system being controlled can react, that is, in the sense of control theory, change its plant d

> control purposes Here you have an implicit assumption that the system being controlled can react, that is, in the sense of control theory, change its plant dynamics, due to the effort of the control. Well, for a lot of work in control, say, in most aerospace applications, that assumption is false or essentially so. Then getting a good metric and using it is fine. E.g., get the power spectrum of ocean wave noise and use that in a control system to keep a submarine at essentially a constant depth. We don't expect that the metric of the ocean wave power spectrum and the control system in the submarine will change the ocean very much! E.g., GPS is a terrific metric, but we don't expect that using it will change the shape of the earth. Etc.