  What ontology does realism about the quantum state suggest? The main extant
view in contemporary philosophy of physics is wave-function realism. We
elaborate the sense in which wave-function realism does provide an ontological
picture; and defend it from certain objections that have been raised against
it. However, there are good reasons to be dissatisfied with wave-function
realism, as we go on to elaborate. This motivates the development of an
opposing picture: what we call spacetime state realism; a view which takes the
states associated to spacetime regions as fundamental. This approach enjoys a
number of beneficial features, although, unlike wave-function realism, it
involves non-separability at the level of fundamental ontology. We investigate
the pros and cons of this non-separability, arguing that it is a quite
acceptable feature; even one which proves fruitful in the context of
relativistic covariance. A companion paper discusses the prospects for
combining a spacetime-based ontology with separability, along lines suggested
by Deutsch and Hayden
