*put sufficiently much into the quantum system that the inclusion of more would not significantly alter practical predictions*

On the other hand, the system *A* should not be so large that *μ* ∪ *A* cannot be described by deterministic quantum mechanics. In the model that we have described, the bifurcation of measurement takes place in the reversible stage of the interaction between *μ* and *A* before irreversibility sets in and fixes the result. In this respect, our analysis is very different from decoherence analysis [11,18].

Beyond *A*, the measurement apparatus must be considered to be an open system with its dynamics described by a Lindblad equation [19]. The starting point for the development here is one of the final states before the Heisenberg cut, i.e., |+, *β*+(*α*)*<sup>A</sup>* or |−, *β*−(*α*)*A*, in one of the sub-ensembles described by *Q*+(*Y*) or *Q*−(*Y*). Thus, the open dynamics continues only in the channel that happens to have been chosen, + or −.

For future work, a more detailed description is needed of a typical *μA*-interaction, including the statistics of the initial states and the selection of one state |0, *α<sup>A</sup>* with a large transition amplitude, leading to a final state (2) with *μ* in one eigenstate, |+*<sup>μ</sup>* or |−*μ*. An important task is to construct a detailed physical model of a non-biased measurement apparatus. The model of Appendix C is a beginning in this respect, but the mathematical assumptions in Equation (3) should be directly tied to physical properties of *A*. In particular, the non-bias property of *A* should be analyzed.

The system *A* should be neither too small nor too large. Then it is reasonable to describe it as mesoscopic, but Bell's principle that we have quoted above gives no indication of its actual size. In the development of realistic models, questions of limits and the accuracy of approximations will have to be handled in more detail.

In practical scientific research, there is a common working understanding of quantum mechanics. Physicists have a common reality concept for a quantum-mechanical system when it is not observed, a kind of pragmatic quantum ontology with the *quantum-mechanical state* of the studied system as the basic concept. Development of this state in time then constitutes the quantum dynamics. If quantum mechanics now can also be used to describe the measurement process, this pragmatic quantum ontology can have a wider validity than has been commonly expected.

**Author Contributions:** Conceptualization, K.-E.E. and K.L.; methodology, K.-E.E.; software, K.L.; formal analysis, K.-E.E. and K.L.; investigation, K.-E.E. and K.L.; writing–original draft preparation, K.-E.E. and K.L.; writing–review and editing, K.-E.E. and K.L.; visualization, K.-E.E. and K.L.; project administration, K.L.

**Funding:** This research received no external funding.

**Acknowledgments:** We thank Erik Sjöqvist and Martin Cederwall for fruitful collaboration in an earlier phase of this project [16]. Financial support from The Royal Society of Arts and Sciences in Gothenburg was important for this collaboration. We are also grateful to Andrew Whitaker for several constructive discussions. We thank two referees, whose critical comments have led us to make several improvements of the paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
