  What is the status of this probability postulate? Is it a law of nature or a contingent fact (if it is a fact at all)? The answers to these questions vary (see Section 7.2.1, Bohm 1953, Valentini 1991a,b, 1992, 1996, 2002, Valentini and Westman 2004, Dürr, Goldstein and Zanghì 1992a,b, 1996, fn. 15, and Callender 2006).