---
title: The Nature of Time
section-id: metaphysics
sort: 150
author: Prof. James Okafor
created: 2026-02-14 09:00
modified: 2026-03-22 10:30
language: en
description: The metaphysics of time: presentism, eternalism, the A-series, and B-series.
---

# The Nature of Time

Time is among the most fundamental features of reality, yet its nature is deeply puzzling. We experience time as flowing—the present moment feels vivid and immediate while the past recedes and the future approaches. But is this experienced flow a feature of time itself, or an artefact of how we experience it? And is the future real? Is the past?

## The A-Series and B-Series

J.M.E. McTaggart distinguished two ways of ordering events in time:

- The **A-series** orders events as past, present, or future. These properties change: what was future becomes present, then past. The A-series involves genuine temporal becoming.
- The **B-series** orders events as earlier than, simultaneous with, or later than. These relations are permanent: if A is earlier than B, this is true at all times. The B-series is static.

McTaggart argued that the A-series is essential to time (since time involves change), but that the A-series is contradictory: every event is past, present, and future, but these are incompatible properties. He concluded that time is unreal.

Most philosophers do not accept the conclusion but take the A-series/B-series distinction as the central organising concept in the metaphysics of time.

## Presentism and Eternalism

**Presentism** holds that only the present moment is real. Past events no longer exist; future events do not yet exist. The past is gone and the future is not here. This matches common sense: we cannot go back to yesterday or visit tomorrow.

Presentism faces difficulties with special relativity. Relativity implies that simultaneity is relative to a frame of reference—there is no frame-independent fact about what is happening ‘now’. If presentism requires a privileged present, it seems in tension with the physics.

**Eternalism** (the block universe view) holds that past, present, and future all equally exist. Time is like space: there is no privileged present any more than there is a privileged ‘here.’ The experienced flow of time is an artefact of our temporal location, not a feature of time itself. Eternalism fits naturally with special relativity.

Eternalism faces the challenge of explaining why the present seems special. If all times equally exist, why do we experience a particular time as ‘now’? And if the future already exists, does this mean the future is fixed? Many eternalists are also determinists, but the connection is not necessary.

**Growing block universe**: Past and present exist, but the future does not yet. The block of reality grows as the present moment advances. This avoids the oddity of a fixed future while maintaining the reality of the past.

## The Flow of Time

Does time flow? We experience it as flowing, but it is difficult to make sense of this scientifically or metaphysically. Flow requires a rate: things flow at so many units per unit of time. But what would it mean for time to flow at so many seconds per second? And what would it mean to flow at a different rate?

**Dynamic theories** (A-theorists) hold that temporal becoming—the flow from future through present to past—is a genuine feature of reality, not merely an appearance. This requires taking the A-series seriously as a real structure, not reducible to B-series relations.

**Static theories** (B-theorists) hold that the apparent flow of time is an artefact of our psychology. We are embedded in time and experience events from a particular temporal location; the ‘flow’ is how this feels from the inside. There is no objective flow: the B-series is all there is.

## Time’s Arrow

The fundamental laws of physics are (largely) time-symmetric: they work equally well run forward or backward. Yet time has a direction: entropy increases toward the future (the second law of thermodynamics), causes precede effects, and we remember the past but not the future.

What grounds the asymmetry? The standard answer appeals to the **past hypothesis**: the early universe was in a state of very low entropy. Given this initial condition and the laws of physics, entropy increases over time, creating the observed arrow. But this raises the question: why was the initial entropy low? This is a deep cosmological question without a settled answer.

## Summary

- McTaggart’s A-series (past/present/future) involves temporal becoming; the B-series (earlier/later) is static
- Presentism holds only the present is real; eternalism holds all times equally exist; the growing block combines these
- The experienced flow of time may or may not reflect a real feature of temporal reality
- Time’s arrow (entropy, causation, memory) requires explanation given the time-symmetry of physical laws
