 Let us define a universal expert function for a given rational agent as one that would guide all of that agent’s probability assignments in this way: an expert function for the agent concerning all propositions. van Fraassen (1984, 1995a), following Goldstein (1983), argues that an agent’s future probability functions are universal expert functions for that agent. He enshrines this idea in his Reflection Principle, where \(P_t\) is the agent’s probability function at time \(t\), and \(P_{t+\Delta}\) is her function at a later time \(t+\Delta\):