 More generally, we might speak of an entire probability function as being such a guide for an agent over a specified set of propositions. Van Fraassen (1989, 198) gives us this definition: “If \(P\) is my personal probability function, then \(q\) is an expert function for me concerning family \(F\) of propositions exactly if \(P(A | q(A) = x) = x\) for all propositions \(A\) in family \(F\).”