The conception of the singular existence of the universe introduced

There is one real universe. This universe may extend indefinitely back in time, in a succession of earlier universes or of earlier states of the universe. We have no sufficient reason to believe in the simultaneous existence of other universes with which we have, and cannot have, now or forever, causal contact.

Causal communion is the decisive criterion for the joint membership of natural phenomena in the same universe. The parts of a universe are causally connected, directly or indirectly, to all the other parts over time. Over time is the first and most important qualification. Two parts of nature belong to the same universe if they share any event in their causal past, even if they have subsequently become causally disjoint. It is the network of causal relations viewed backward into the past that determines the scope of causal communion and thus the separate existence of a universe. The criterion is dynamic rather than static, historical rather than exclusively structural, and presupposes the reality of time.