OBJECT SUBJECT SPLIT AND SUPERSELECTION PARTIAL STATES

In an accompanying work it was shown that any subset of quantum mechan ical observables determines an (up to equivalence) unique partial stat istical theory with partial states. The crucial role of a so-called ba sic (e.g., pointer) observable in determining an object-subject split with a well-defined subject was made clear. In this article the subjec t subsystem is assumed to be quantum mechanical, but such that the bas ic observable is a superselection one. This leads to superselection pa rtial states and to a different approach to the split. Advantages and disadvantages of the latter approach are discussed.