Countable additivity is not as innocent as it looks: it rules out the possibility that any agent is indifferent over a countably infinite set of mutully exclusive possibilities. De Finetti (1970, 1972) famously argued that we ought to reject countable additivity since it is conceivable that God could pick out a natural number “at random” and with equal (zero) probability. For another example, suppose you assign 50% credence to the proposition \(\neg B\) that not all ravens that will ever be obse