So the necessity of the past may simply be the principle that past events are outside the class of causable events. There is a temporal asymmetry in causability because everything causable is in the future. But some of the future is non-causable as well. Whether or not determinism is true, there are some events in the future that are causally necessary. If a future event E is necessary, it is causable, and not E is not causable. But if the necessity of the past is the non-causability of the past, it would be odd to pick out the class of propositions about the past as possessing an allegedly di...