Introduction

Of ultimate concern to the theory of policy, as conceived in this book, is the operation of instruments on targets. For the dynamic theory of policy, the linear system representation interposes the system state between the instruments as inputs and the targets as outputs. Chapter 5 has just finished examining the operation of the instruments on the states. Chapter 6 now completes the link from instruments to targets by examining the operation of the states on the targets. Just as Chapter 5 associates various controllability properties with the ability of the instruments to affect the states, in similar fashion Chapter 6 associates various observability properties with the ability of the states to affect the targets.

But whereas state controllability is an existence property, state observability is a uniqueness property. The question to be studied is not, as in Chapter 5, whether the instruments can be adjusted intertemporally to effect an arbitrary state transfer; but rather, given that a particular intertemporal target transfer has occurred, is there a unique intertemporal state transfer responsible for this target transfer? Why such a uniqueness question is of recurring importance, and precisely how it pairs with the existence question, are topics to be resolved during this chapter.

Some heuristic motivation can be provided with the benefit of hindsight.