  Intuitively, \(S^*\) is a measuring instrument for an observable \(A\) just in case there is some observable feature of \(S^*\) (it doesn’t matter what, just something whose values can be ascertained by looking at the device), which is correlated with the \(A\)-values of systems fed into it in such a way that we can read those values off of \(S^*\)’s observable state after the interaction. In philosophical parlance, \(S^*\) is a measuring instrument for \(A\) just in case there is some observable feature of \(S^*\) which tracks or indicates the \(A\)-values of systems with which it interacts in an appropriate way. 