  It is very nearly a piece of philosophical orthodoxy that non-trivial objective chances require indeterminism. The view is seldom defended; even those who trouble to state the view explicitly (Lewis, 1980: 120) don’t go to the further effort of justifying it, perhaps because it seems obvious. After all, under determinism, someone who knew the past and the laws would be in a position to know with certainty every future outcome. So our use of probability under determinism must be purely subjective, a side-effect of our (perhaps unavoidable) ignorance of the past or the laws. If this orthodoxy is correct, at least the left-to-right direction of P1 would be true.