 For Rundle, the past and the future are symmetrical; it is only our knowledge of them that is asymmetrical. Any future event lies at a finite temporal distance from the present. Similarly, any past event lies at a finite temporal distance from the present. For each past or future event, beginning from the present, there can always be either a prior past event or a subsequent future event. Hence, for both series an infinity of events is possible, and, as symmetrical, the infinity of both series is the same. Since the series of future events is not an actual but a potential infinite (or, better, an “indefinitely extendible” series, 2004: 168; Craig and Sinclair 2012, 104–5), the series of past events is also indefinitely extendible. It follows that although the future is actually finite, it does not require an end to the universe, for there is always a possible subsequent event (2004: 180). Similarly, although any given past event of the universe is finitely distant in time from now, a beginning or initial event can be ruled out; for any given event there is a possible earlier event. However, since there is a possible prior or possible posterior event in any past or future series respectively, the universe, although finite in time, is temporally unbounded (indefinitely extendible); both beginning and cessation are ruled out. [How Rundle (2004: 176–78) gets from the possibility of a subsequent event to actually ruling out cessation and beginning is unclear.] Since there is no time when the material universe did not exist, it is not contingent but necessary. Hence, although the principle of sufficient reason is still true, it applies only to the components of the material universe and not to the universe itself. No explanation of the universe is possible. The universe, as matter-energy, is neither caused nor destructible, not in the sense that it could have been caused or could cease, but in the sense that “the notions of beginning and ceasing to exist are inapplicable to the universe” (2004: 178).