The everyday world is a temporal world: the signing of the Declaration of Independence was later than the Lisbon earthquake; the Cold War is in the past; your death is in the future. There is no getting away from time.

The ontology of time is currently dominated by two theories: Presentism, according to which “only currently existing objects are real,” and Eternalism, according to which “past and future objects and times are just as real as currently existing ones.” In my opinion, neither Presentism nor Eternalism yields a satisfactory ontology of time. Presentism seems both implausible on its face and seems in conflict with the Special Theory of Relativity, and Eternalism gives us no handle on time as universally experienced in terms of an ongoing now. (There is a third theory, the Growing Block Universe, according to which the past is real but the future is not; but it also conflicts with the Special Theory of Relativity.) So, I shall by-pass these theories for now and return to them later.

This chapter aims to develop a way to understand time that is adequate both to physics and to human experience. It begins with McTaggart's framework of the A-series and the B-series – the framework that underlies both Presentism and Eternalism. I shall set out a theory (that I call “the BA theory”) that shows how the A- and B-series are related without reducing either to the other.