 It is also worth asking whether time must be represented by a single line. Perhaps we should take seriously the possibility of time’s consisting of multiple time streams, each one of which is isolated from each other, so that every moment of time stands in temporal relations to other moments in its own time stream, but does not bear any temporal relations to any moment from another time stream. Likewise we can ask whether time could correspond to a branching line (perhaps to allow for the possibility of time travel or to model an open future), or to a closed loop, or to a discontinuous line. And we can also wonder whether one of the two directions of time is in some way privileged, in a way that makes time itself asymmetrical. (We say more about this last option in particular in the section on time and physics.)