  QBism has long recognized quantum states, POVM elements, Kraus operators, and
even unitary operations to be cut from the same cloth: They express aspects of
an agent's personal belief system concerning the consequences (for her) of
actions she might take on her external world. Such action-consequence pairs
have conventionally been called "quantum measurements." The calculus of quantum
theory is then viewed as an empirically motivated addition to Bayesian decision
theory when brought to this notion of measurement. This radical approach has
allowed QBism to eliminate conceptual problems that plague other
interpretations of quantum mechanics. However, one issue has remained elusive:
If a QBist does not believe in the existence of an ontic (agent-independent)
dynamical variable evolving over time, why would there be any constraints on
her quantum-state assignment in the absence of performing a measurement? Why
would she introduce unitary or open-system quantum dynamics at all? Here, we
present a representation theorem based on van Fraassen's reflection principle
to answer these questions. Simply put, an agent's assignment of quantum
dynamics represents her belief that a measurement action she is contemplating
would not change her current odds for future gambles. A corollary to this
approach is that one can make sense of "open-system dynamics" without
introducing an "environment with a measurement record" as is common in
decoherence accounts of quantum measurement. Instead, the QBist understanding
of decoherence rests entirely on an agent's beliefs about the system of
interest (not system plus environment) and her judgments about measurements she
might perform on that system. More broadly, this result establishes QBism's
contention that measurement itself is the central concept of quantum theory and
thus the framework upon which any future QBist ontology must hang.
