 We may go still further. There may be universal expert functions for large classes of rational agents, and perhaps all of them. The Principle of Direct Probability regards the relative frequency function as a universal expert function for all rational agents; we have already seen the importance that proponents of calibration place on it. Let \(A\) be an event-type, and let relfreq\((A)\) be the relative frequency of \(A\) (in some suitable reference class). Then for any rational agent with probability function \(P\), we have (cf. Hacking 1965):