 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 distinct kind of necessity since some of the future has that same kind of necessity.