Alan Hájek delivers a devastating blow against frequentism and other theories of probability in his influential 2007 paper “The reference class problem is your problem too.” In brief, when a hypothesis H or proposition P can be classified in various ways, and when the probability of H or P varies depending on how it is classified, the reference class problem has reared its ugly head. For his part, Hájek defines the reference class problem formally as follows:
“[A hypothesis H or proposition P] may be classified as belonging to set S1, or to set S2, and so on. Qua member of S1, its probability is p1; qua member of S2, its probability is p2, where p1 ≠ p2; and so on. And perhaps qua member of some other set, its probability does not exist at all” (Hájek, 2007, p. 565).
In his reference-class paper, Hájek carefully and lucidly explains why frequentism and other leading interpretations of probability — all theories of probability, that is, except for “radical subjectivism” or pure Bayesian probability — are contaminated by the reference class problem (ibid., pp. 566-580). Nevertheless, although subjectivism is immune from the reference class problem, Hájek attempts to discredit Bayesian methods. His critique of subjectivism, in summary, is that it is atheoretical — it is a “no-theory theory of probability” (ibid., p. 577). According to Hájek, “no-theory theories of probability” are apparently bad because they “leave quite obscure why probability should function as a guide to life, a suitable basis for rational inference and action” (ibid., p. 564). But why is radical subjectivism a “no-theory theory of probability” in Hájek view? Because, according to Hájek, radical subjectivists are, well, too subjective! Continue reading →