  How should we interpret physical theories, and especially quantum theory, if
we drop the assumption that we should treat it as an exact description of the
whole Universe? I expound and develop the claim that physics is about the study
of autonomous, but not necessarily isolated, dynamical systems, and that when
applied to quantum mechanics this entails that in general we should take
quantum systems as having mixed states and non-unitary dynamics. I argue that
nonetheless unitary dynamics continues to have a special place in physics, via
the empirically-well-supported reductionist principles that non-unitarity is to
be explained by restriction to a subsystem of a larger unitary system and that
microscopic physics is governed by unitary and largely known dynamics. I
contrast this position with the `Open Systems View' advocated recently by
Michael Cuffaro and Stephan Hartmann.
