Time, metaphysics of

1. The A-theory and the B-theory

The debate between these theories emerged out of attempts to do justice to some of our deeply-held intuitions about the nature of time. One such intuition is that the future is unreal. What must time be like to accommodate that intuition? Another consideration is that concerning the relationship between our temporal representations and the temporal reality they represent. Much of our ordinary language is tensed. That is, it appears to refer to events that are located in the past, present or future. Does this have any significance for whether events really are located in the past, present or future?

Both theories recognise that, at the level of ordinary thought and talk, there are two different ways in which we conceptualise the ordering and location of events and moments in time. They disagree over their metaphysical significance. Consider, for a moment, the entire temporal history of the universe. It consists of a series of events, stretching from the Big Bang, past the formation of the planets in our solar system, the evolution of life, events in the 20th century, events that are going on now, and on into the unknown future.

One way of thinking about this series involves the recognition that each event is located in either the past, present or future, and that they can be ordered according to whether they are in the distant past, near past, present, near future, or distant future. The series of events conceived in this way is called the A-series, a usage derived from McTaggart (1927).

The other way involves the recognition that each event in this series is temporally related to every other event, in that it is either earlier than, simultaneous with, or later than it. Both the temporal locations of events relative to each other, and their temporal ordering, can be given in terms of these temporal relations, without any need to employ the concepts of pastness, presentness and futurity. The series of events conceived in this way is called the B-series.

The A-theory of time is so-called because, according to it, A-series concepts are essential for an adequate characterisation of time. While it recognises that events stand in temporal relations to each other, and so constitute a B-series, it holds that this is insufficient to characterise the true nature of time. The A-theory is also called the tensed theory. In the context of the metaphysics of time, the term ‘tense’ refers not to the familiar linguistic phenomenon, but to the A-series location of an event. A tensed attribute, such as ‘three hours ago’, ‘fifty years hence’, or ‘today’, when ascribed to an event, assigns it a position in the A-series. According to the A-theory these concepts are essential for an adequate description of temporal reality. The metaphysical nature of time is such that there are tensed facts: facts about the pastness, presentness or futurity of events (see Continuants §2). The A-theory is also described as a dynamic theory of time because, once a real distinction between past, present and future is recognised, it must also be recognised that events continually change with respect to their pastness, presentness or futurity.

According to the B-theory of time, the B-series ordering of events is sufficient for an adequate metaphysical characterisation of time. Although B-theorists recognise that the distinction between past, present and future is one that we constantly allude to in everyday life, this says more about us and our epistemic access to temporal reality than it does about the metaphysical nature of time. Our use of tensed concepts merely reflects the fact that, when we use them, we do so from a particular temporal point of view. If I describe an event as present, I do so at a particular time, and what I say is true if, and only if, the event happens simultaneously with my description of it. Similarly, if I describe an event as past (future), my description is apt if, and only if, the event is temporally located earlier (later) than the time at which I so describe it. No tensed facts are needed to ground the truth of any true description that employs a tensed concept. That job can be done by the tenseless temporal relations that events stand in to each other. Consequently, the B-theory of time is also called the tenseless theory of time, because according to it, time is essentially tenseless (see Continuants §2).

The B-theory is also described as the static, or block universe theory of time. Since it rejects the distinction between past, present and future, it also rejects the associated view that time is essentially dynamic. If nothing is really past, present or future, then nothing can change from being future to present to past. Furthermore, the temporal relations that events stand in to one another are unchanging. If e1 occurs earlier than e2, then the fact that those two events are so related will never change. Recall the picture of the temporal history of the universe, as understood by the B-theory. According to that picture, every actual event is located somewhere in that series, and it never changes its temporal location. The picture generated is one of a four-dimensional block of spacetime, with every event permanently occupying its own temporal location. The B-theory does not deny that ordinary change occurs. Several theories of ordinary change are consistent with it. What it does deny is that there is any sense in which time itself changes (see Change §2).

Can’t they both be right? Why do we have to choose between these two theories? The reason we have to choose is because the issue rests on whether or not the A-series is a constituent of temporal reality. Anyone who thinks that time is constituted by both an A-series and a B-series is an A-theorist.

One way of capturing the distinction between the A-theory and the B-theory is to ask, what would temporal reality be like if sentient beings had never evolved? According to the A-theory, if there were no observers there would still be an objective distinction between past, present and future. One moment in the entire temporal history of the universe would be the present moment, and which moment that was would be constantly changing. According to the B-theory, if there were no observers there would be no present moment.

To summarise, both theories agree that our ordinary, everyday conception of time involves both A-series and B-series concepts. The A-theorist thinks the A-series concepts describe the true nature of time. There is an objective, observer-independent distinction between past, present and future, and how this distinction carves up the series of events is constantly changing. The B-theorist denies that the distinction between past, present and future is a feature of the way time is in itself, and maintains that what it is for time to exist is for events to stand in temporal relations to each other.