This Chapter develops a realist information-theoretic interpretation of the
nonclassical features of quantum probabilities. On this view, what is
fundamental in the transition from classical to quantum physics is the
recognition that \emph{information in the physical sense has new structural
features}, just as the transition from classical to relativistic physics rests
on the recognition that space-time is structurally different than we thought.
Hilbert space, the event space of quantum systems, is interpreted as a
kinematic (i.e., pre-dynamic) framework for an indeterministic physics, in the
sense that the geometric structure of Hilbert space imposes objective
probabilistic or information-theoretic constraints on correlations between
events, just as the geometric structure of Minkowski space in special
relativity imposes spatio-temporal kinematic constraints on events. The
interpretation of quantum probabilities is more subjectivist in spirit than
other discussions in this book (e.g., the chapter by Timpson), insofar as the
quantum state is interpreted as a credence function---a bookkeeping device for
keeping track of probabilities---but it is also objective (or intersubjective),
insofar as the credences specified by the quantum state are understood as
uniquely determined, via Gleason's theorem, by objective correlational
constraints on events in the nonclassical quantum event space defined by the
subspace structure of Hilbert space.
