  A persistent focus on the concept of emergence as a core element of the
scientific method allows a clean separation, insofar as this is possible, of
the physical and philosophical aspects of the problem of outcomes in quantum
mechanics. The philosophical part of the problem is to explain why a closed
system has definite experimental outcomes. The physical part is to show
mathematically that there exists a limit in which the contradiction between
unitary Schroedinger dynamics and a reduction process leading to distinct
outcomes becomes negligible according to an explicitly stated criterion, and to
make this criterion as objective as possible. The physical problem is solved
here by redefining the notion of a quantum state and finding a suitable measure
for the change of state upon reduction. The appropriate definition of the
quantum state is not as a ray or density operator in Hilbert space, but rather
as an equivalence class consisting of all density operators in a given
subspace, the members of which all describe the same experimental outcome. For
systems containing only subsystems that are integrated with their environments,
these equivalence classes can be represented mathematically by projection
operators, and the resulting formalism is closely related to that used by von
Neumann to study the increase of entropy predicted by the second law of
thermodynamics. However, nearly isolated subsystems are reduced only
indirectly, as a consequence of their interaction with integrated subsystems.
The reduced states of isolated subsystems are the same conditional states used
in the definition of quantum discord. The key concepts of decoherence theory
can all be adapted to fit this definition of a quantum state, resulting in a
unified theory capable of resolving, in principle, all aspects of the quantum
measurement problem. The theory thus obtained is weakly objective but not
strongly objective.
