---
title: The Nature of Time
sort: 150
section-id: metaphysics
description: A-series and B-series, presentism, eternalism, and the growing block theory of time.
language: en
---

# The Nature of Time

Time is both utterly familiar and deeply puzzling. We live in it, measure it, experience its passage. Yet when we ask what time is, whether the past and future exist, whether time flows or merely seems to, we find ourselves quickly in some of philosophy's most difficult territory.

## McTaggart's A-Series and B-Series

J.M.E. McTaggart introduced the most influential framework for the philosophy of time in his 1908 paper "The Unreality of Time" ^[McTaggart, J.M.E., "The Unreality of Time", *Mind* 17, 1908, pp.457-474].

The *B-series* is the ordering of events as earlier, simultaneous, and later. Every event stands in a fixed B-relation to every other: the Battle of Hastings is earlier than the French Revolution; your birth is earlier than your reading this sentence. These relations are *permanent*: if A is earlier than B, it always has been and always will be.

The *A-series* orders events as *past*, *present*, and *future*. Unlike B-relations, A-properties change: what is now future becomes present and then past. The event of your reading this sentence was future, is now present, and will be past.

McTaggart argued that the A-series is essential to time — without the genuine distinction between past, present, and future, there would be no temporal becoming, no flow of time, and time would not be genuinely real. But the A-series is contradictory: every event has all three properties (past, present, future), which are mutually exclusive. Attempts to resolve the contradiction by saying "the event is present *now*, past *at later times*, future *at earlier times*" invoke further temporal moments and generate a vicious infinite regress. McTaggart concluded that time is unreal.

Most philosophers reject the conclusion but accept that McTaggart identified a genuine structural puzzle.

## Presentism

*Presentism* holds that only present entities exist ^[Crisp, T., "Presentism", in *Oxford Handbook of Metaphysics*, 2003]. The past is gone; the future is not yet here. "There were dinosaurs" is true, but dinosaurs do not now exist in any sense — they existed and no longer do.

Presentism is the view that most naturally fits everyday temporal experience. The past seems gone; the future seems open.

**The problem of cross-temporal relations:** Many true claims seem to relate present entities to past ones: Caesar crossed the Rubicon *before* you were born. If Caesar does not now exist, what makes this claim true? Presentists have responded with *truthmakers* that exist presently — facts about the present state of the world — and with *temporal ersatzism* (abstract representations of past times).

**Reconciling presentism with special relativity:** Relativity implies there is no absolute simultaneity — different reference frames carve the four-dimensional spacetime differently. This is deeply problematic for presentism, which requires a distinguished present ^[Putnam, H., "Time and Physical Geometry", *Journal of Philosophy* 64, 1967].

## Eternalism (The Block Universe)

*Eternalism* — the view associated with Einstein's spacetime — holds that past, present, and future entities all equally exist, merely at different temporal locations ^[Sider, T., *Four-Dimensionalism*, Oxford UP, 2001, ch.2]. The universe is a four-dimensional "block" in which all events co-exist; the distinction between past, present, and future is merely perspectival, like the distinction between here and there.

Eternalism accommodates special relativity naturally: there is no privileged present, and the temporal order of events can be frame-relative.

**The problem of temporal passage:** Eternalism seems to make temporal experience mysterious. If past and future are equally real, why does time seem to pass? Why do we experience becoming rather than merely co-existing with our past and future selves in a static four-dimensional block?

Some eternalists accept this implication and argue that the *passage* of time is an illusion — a product of our temporal perspective, not a feature of reality. Others have argued that passage can be accommodated within an eternalist framework through the *moving spotlight* theory.

## The Growing Block Theory

C.D. Broad proposed an intermediate view: the growing block ^[Broad, C.D., *Scientific Thought*, 1923, pp.66-68]. The past and present are real and fixed; the future does not yet exist. As time passes, new events come into existence and the block grows. This preserves the reality of temporal becoming without committing to a privileged present.

**Objections:** If the growing block is correct, and past moments are real and fixed, how do we know we are in the present rather than in some past moment (which is, after all, equally real)? This generates a peculiar form of scepticism about our temporal location ^[Tooley, M., *Time, Tense, and Causation*, Oxford UP, 1997].

## The Direction of Time

Physics, at the fundamental level, is largely time-symmetric: the laws of mechanics, electromagnetism, and quantum mechanics (with minor exceptions) work equally in both temporal directions. Yet our experience of time is strongly asymmetric: we remember the past and not the future; causes precede effects; entropy increases.

Boltzmann argued that the thermodynamic arrow of time — the increase of entropy — explains the asymmetry ^[Boltzmann, L., *Lectures on Gas Theory*, 1896-98]. We are in a low-entropy region of a vastly larger, mostly high-entropy universe; the Second Law of Thermodynamics is a statistical fact about macroscopic systems, not a fundamental law.

The *causal theory* of time holds that the direction of time is grounded in the direction of causation: the future is the direction in which causal processes flow. But if causation itself is time-asymmetric, this seems circular.

**Temporal experience:** Husserl's phenomenology of time-consciousness distinguished *retention* (the immediate just-past), *primal impression* (the now), and *protention* (the immediate just-future) as three interlocking structures of temporal awareness ^[Husserl, E., *On the Phenomenology of the Consciousness of Internal Time*, 1928]. The experience of time as flowing may be constituted by this structure rather than being evidence of metaphysical flow.

The nature of time remains one of the most contested and most fundamental questions in metaphysics, connecting to physics, the theory of causation, personal identity, and the phenomenology of experience.
