Category: time-and-causality
Type: argument
Associated thinker(s): J. M. E. McTaggart
Original publication or approximate period: McTaggart, “The Unreality of Time” (Mind, 1908)
Difficulty: Advanced
Themes: A-series and B-series, change, tense
Related experiments: The Grandfather Paradox; The Twin Paradox
McTaggart distinguishes two ways of ordering times. The A-series orders them as future, present, and past; an event that is future will later be present and then past. The B-series orders times by relations such as earlier than and later than, which do not change.
He argues that time requires genuine change, and that change requires the A-series: B-series relations alone are fixed. He then argues that the A-series is contradictory. Past, present, and future are incompatible, yet every event has all three in succession—first future, then present, then past. He considers the reply that an event was future, is present, and will be past, but says that explaining these tenses presupposes time and generates a regress. He concludes that time is unreal.
Does genuine change require objective tense? Can an event be future, present, and past without contradiction?
The argument remains central to debates about time and change. B-theorists hold that change can be described through earlier–later relations; A-theorists hold that at least some important forms of change require objective past, present, and future distinctions. There is a range of views within each broad family.
The debate also intersects with physics. The relativity of simultaneity creates a challenge for A-theories, especially presentism, but it does not by itself settle the metaphysical debate: the physical result and the additional assumption that no reference frame is privileged can be distinguished.
B-theory explains change in terms of before-and-after relations and denies that objective tense is required for real change. A-theories retain objective tensed facts, but differ over how to answer McTaggart’s contradiction argument and which events exist. Presentism and the growing-block view are distinct A-theoretic positions, not one shared solution. McTaggart’s own conclusion is that time is unreal.
McTaggart is not arguing that clocks fail; his conclusion concerns the metaphysics of time. His contradiction argument is also not simply that each event is past, present, and future simultaneously. He says the incompatible A-properties apply successively, then argues that the tensed reply—“was future, is present, will be past”—presupposes the time it is meant to explain and leads to circularity or an infinite regress. Later A-theorists need not accept that his reply defeats every tensed theory.
The three-premise summary above is one standard reconstruction, not the only way to present McTaggart’s argument. Later discussion includes different reconstructions and replies; the key issue is which premise or inference each view rejects.
McTaggart’s original article was read in a full-text scan reproduction; relevant passages include the A/B distinction and change argument (pp. 458–465), and the contradiction, tensed reply, and regress (pp. 468–470). The SEP entry on McTaggart (§3) presents one three-premise reconstruction and notes that the best reconstruction of his contradiction argument is contentious. The SEP entry on time (§§4–6) distinguishes the A- and B-theory families and their range of views; its discussion of relativity explains why the connection is not a simple refutation of A-theory. This summary is simplified and does not claim to settle the argument. It is an original educational explanation, not expert review or an exhaustive survey.
Research check: 2026-11-01. The sources and passage locations consulted for this review are recorded in the local evidence ledger.