 Some subjectivists impose the further rationality requirement of regularity: anything that is possible (in an appropriate sense) gets assigned positive probability. It is advocated by authors such as Jeffreys (1939/1998), Kemeny (1955), Edwards et al. (1963), Shimony (1970), and Stalnaker (1970). It is meant to capture a form of open-mindedness and responsiveness to evidence. But then, perhaps unintuitively, someone who assigns probability 0.999 to this sentence ruling the universe can be judged rational, while someone who assigns it probability 0 is judged irrational. See, e.g., Levi (1978) for further opposition to regularity.