Theories of Time

Ask someone what time is and they will usually say that it passes. The morning becomes the afternoon, what was ahead of you is now behind you, and nothing you do can stop the flow. Ask a physicist to write down the passing and there is nothing in the equations to point at: they describe a world laid out along a time dimension, with every moment present in the description. Metaphysics of time begins with that gap. One side holds that the passing is a real feature of the world and any theory that omits it is incomplete. The other holds that the passing is a feature of how we experience a world that is, in itself, simply spread out in time.

Two ways of ordering events

Events can be put in order in two quite different ways, and the whole debate depends on seeing the difference. The distinction comes from J. M. E. McTaggart's paper "The Unreality of Time" (Mind, 1908).

The first ordering uses the positions past, present and future. The Battle of Hastings is past, this sentence is present, and the next general election is future. These positions change: the election, now future, will be present and then past. McTaggart called the series of events ordered this way the A-series.

The second ordering uses relations between events: earlier than, later than, and simultaneous with. Hastings is earlier than the Armada, which is earlier than the moon landing. These relations do not change. If Hastings is earlier than the Armada, it was always so and always will be. McTaggart called this the B-series.

A B-theorist does not deny that people say "it is raining now". The claim is that such a sentence is made true by a relation between the rain and the saying of it, much as "it is raining here" is made true by the rain's relation to the speaker's place. No spot in space is the objectively privileged "here", and on this view no moment is the objectively privileged "now".

The two theories give different accounts of change. Recall that change is a persisting thing having a property at one time and lacking it at another. That definition needs no tense in it. The poker is cold at noon and hot at one o'clock, and both facts are B-series facts. The B-theorist can therefore say that change is exactly what the definition says and nothing more. McTaggart disagreed, and his reason is the argument that has structured the subject since.

McTaggart's argument

McTaggart did not set out to defend an A-theory. His conclusion is stronger: there is no time at all. The argument has two stages.

The second premise is the one the B-theorist rejects, and it is worth seeing why McTaggart believed it. Take the death of Queen Anne. That it is earlier than the French Revolution is true at every time. It does not become false. If the event's temporal properties were only of this kind, nothing about it would ever differ from one time to another, and so nothing about it would change. The only property of the event that is ever different is how far in the past it lies.

The third premise is the surprising one. The event is past, so it is not present or future. But every event that is past was once present and was before that future. So each event has all three positions. The obvious reply is that it has them at different times: it is future, then present, then past, and there is no clash.

McTaggart's answer is that this reply uses the very thing at issue. To say the event is future, will be present and will be past is to describe it with positions in a further series, a series of times at which it has each position. Each of those times is itself future, present or past, so each has all three positions, and the same reply must be made again, and so on without end. The explanation of the contradiction brings in a new case of it. This regress is vicious, since no stage in it ever removes the contradiction.

There are two main ways out. The A-theorist can treat the tensed positions as primitive and refuse to analyse "is future" as "is future at some time", so that there is no second series to start the regress. This is the course taken by A.N. Prior, the founder of tense logic, who argued in Past, Present and Future (1967) that tense is basic and cannot be defined in terms of dates. Or the A-theorist can accept that only one position is really instantiated, so that the world has just one present and nothing is ever both future and past. That second option is the one the three metaphysical views below develop in their different ways.

Presentism

Presentism is the plainest A-theory. It treats the present as what there is. Becoming is easy to state on it: what exists changes, as things go out of existence and others come into existence. The passage of time is the changing of what is real.

The case for it begins with the ordinary sense that the past is gone. Prior gave it a sharper form in his paper "Thank Goodness That's Over" (Philosophy, 1959). When a painful ordeal has ended, one is relieved, and the relief is directed at a fact that is not a date. Relief at learning that the ordeal ended at a given hour would make no sense unless one also knew that the hour is now past. What one is glad about is that the ordeal is over, and being over is a tensed fact. A B-theorist who translates the thought into dates leaves out the thing that made it worth feeling.

The standard objection is about what makes statements about the past true.

A second difficulty is about relations across time. "Caesar was more ambitious than anyone now alive" relates Caesar to something. If neither relatum is present, the relation holds between nothing. Presentists respond by paraphrase, treating such sentences as claims about how the present world was, but the paraphrases get complicated quickly. There is a third difficulty from physics, which asks whether any single moment is "the present" at all. It belongs to a later rung, and the point to carry from here is only that presentism is the view under most pressure from it.

Eternalism

Eternalism is a thesis about what exists. It does not say how the time we are in is distinguished, if it is. That separates it from the B-theory, and the two views are often run together.

The eternalist picture is of a four-dimensional world, in which events at every time lie along a time dimension as places lie along a road. The dinosaurs are no more gone than Paris is gone when you are in Rome. They are elsewhere in the dimension, and your use of "now" picks out your own location, as "here" does. Quine, Smart and, more recently, Theodore Sider in Four-Dimensionalism (2001) have defended this view. It fits the earlier discussion of persistence. If things have temporal parts, as the perdurantist says, then a person is a four-dimensional whole extended through time, and the earlier and later parts are as real as the present one. Eternalism does not require perdurantism, since an endurantist can hold that things are wholly present at each of the times at which they exist and still treat all those times as real. But the pairing is natural, and it is the form most often defended.

The argument for eternalism is mostly one of economy and fit. It makes the truth of past-tensed and future-tensed claims no more mysterious than the truth of claims about distant places, so it faces no truthmaker problem. It treats reference to the past, and the relations between Caesar and us, as ordinary. And it matches the structure of spacetime in physics, where no moment is marked as the real one.

The growing block

C. D. Broad proposed this in Scientific Thought (1923), and Michael Tooley developed it in Time, Tense and Causation (1997). Its attraction is that it tries to keep the strengths of both rivals. Like the eternalist, it holds that the past is real, so there is no truthmaker problem for "there were dinosaurs": the dinosaurs are still in the block. Like the presentist, it holds that the future is unreal and that the present is special, so becoming is easy to state. Becoming is the coming into being of new reality, and the present is where that happens.

This asymmetry matches a common thought. The past is fixed and the future open. We cannot change what has happened, and what will happen has not yet been settled. On the growing block that is a fact about existence rather than about our knowledge.

There is a second complaint, about the shape of the theory. The block grows along one dimension, but nothing in the theory explains why the line between existent and non-existent should fall at the present rather than somewhere else. Presentism and eternalism each have a principled stopping point: nothing beyond now, or no stopping point at all. The growing block has an asymmetry that it asserts and does not explain.

What the choice costs

None of the three views is free. Each keeps something and pays for it.

Presentism keeps the plain idea that the present is real and the rest is not. It pays with the truthmaker problem and with relations across time. Eternalism keeps the clean treatment of the past and future as ordinary reality and fits the physics. It pays with the feeling that something has been left out: that the passing of time, and the special status of now, have been explained away rather than explained. The growing block takes the asymmetry between the past and the future seriously, and it pays with the sceptical problem and the unexplained edge.

The B-theory and the A-theory also differ in a way that cuts across these three. A-theories must find a place for the privileged present in the physics, and special relativity makes that hard, since whether two distant events are simultaneous depends on the frame of reference one uses. A single objective present across the universe is exactly what relativity treats as unavailable. A-theorists reply by treating the preferred present as a feature of the world that physics does not capture, or by looking for a privileged frame; both moves are taken up in the advanced tier. B-theorists avoid the difficulty but have to say what is wrong with Prior's relief and with the sense that time really does pass.

A useful way to hold the debate is to ask what is being explained. If it is the structure of events in time, the B-theory is enough. If it is our experience of time, the A-theorist cites becoming and the B-theorist owes a story about why passage appears to happen.

Each of these views treats the order of events as a given, and the next article asks what the relation is that orders them as causes and effects.