  In this paper we consider theories in which reality is described by some
underlying variables. Each value these variables can take represents an ontic
state (a particular state of reality). The preparation of a quantum state
corresponds to a distribution over the ontic states. If we make three basic
assumptions, we can show that the distributions over ontic states corresponding
to distinct pure states are non-overlapping. This means that we can deduce the
quantum state from a knowledge of the ontic state. Hence, if these assumptions
are correct, we can claim that the quantum state is a real thing (it is written
into the underlying variables that describe reality). The key assumption we use
in this proof is ontic indifference - that quantum transformations that do not
affect a given pure quantum state can be implemented in such a way that they do
not affect the ontic states in the support of that state. In fact this
assumption is violated in the Spekkens toy model (which captures many aspects
of quantum theory and in which different pure states of the model have
overlapping distributions over ontic states). This paper proves that ontic
indifference must be violated in any model reproducing quantum theory in which
the quantum state is not a real thing. The argument presented in this paper is
different from that given in a recent paper by Pusey, Barrett, and Rudolph. It
uses a different key assumption and it pertains to a single copy of the system
in question.
