Ontological parsimony and the open past

A history is a complete possible temporal evolution of the universe. There are two main conceptions of histories: Lewis’s divergence, in which distinct histories are all disjoint, and the standard branching time (BT) conception, in which histories overlap and branch towards the future (‘open future’). As Greg Restall noted, if a realist conception of histories is adopted, considerations of ontological parsimony favor the standard BT conception over Lewis’s divergence. In this note, I observe that perfectly analogous considerations can be raised against the standard BT conception and in favor of a non-standard, ‘open past’ BT conception, in which histories are allowed to branch both towards the future and the past.