 This leads to a deeper problem in the idea of the necessity of the past. Zagzebski (2014) argues that the interpretation of the necessity of the past as a purely temporal modality is confused. What people generally mean by the necessity of the past is that the past is causally closed, meaning the past is neither causable nor preventable. Understood that way, the necessity of the past is not a purely temporal modality, and it is not a form of necessity. The categories of causability and non-causability do not correspond to the standard modal categories of the necessary, possible, and impossible. The attempt to assimilate the causal categories to modal categories is a mistake.