02 · QUANTUM BAYESIANISM
Probability & the Born rule.
Personal probabilities, constrained by quantum theory.
Belief has a mathematical discipline.
In personalist Bayesian probability, a probability records an agent’s degree of belief. Coherence constrains how those beliefs fit together: assigning 0.7 to an event requires assigning 0.3 to its complement.
For mutually exclusive, exhaustive outcomes, the probabilities sum to one. Those bookkeeping rules alone do not recover quantum theory.
The quantum addition.
Fuchs and Schack interpret the Born rule as an additional normative constraint. It connects the probabilities an agent assigns to different possible measurement actions, rather than deriving probabilities from an objective quantum state.
Here ρ is a density operator and Ej a measurement effect. The formula is shared quantum mathematics; its QBist reading concerns what those assignments mean.
Follow the argument.
Fuchs & Schack · Quantum-Bayesian Coherence (2013) ↗The paper develops the constraint using probabilities for a reference measurement.