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The philosophy of time investigates the fundamental nature of temporal reality: Is the past real? Does time genuinely pass, or is this an illusion? What distinguishes the past from the future if both are equally real? McTaggart's 1908 paper introduced the A-series (past/present/future distinctions that change) and B-series (earlier/later relations that are fixed) as competing ways of ordering time, and argued both are defective — concluding controversially that time is unreal. The subsequent debate has centered on whether temporal passage is objective (dynamic theories) or a cognitive projection onto a static four-dimensional manifold (static block-universe theories).
Read McTaggart's 'The Unreality of Time,' then Le Poidevin's Travels in Four Dimensions for a comprehensive introduction. Map each view onto its commitments about the ontological status of past and future events.
When you think about time, you probably take for granted two different ways of locating events. You might say "the lecture happened last Tuesday" — locating it relative to now, using the shifting categories of past, present, and future. Or you might say "the lecture happened before the exam" — a fixed relation that never changes regardless of when you are saying it. J.M.E. McTaggart formalized this distinction in 1908 as the A-series and B-series, and his paper set the agenda for analytic philosophy of time for the next century.
The B-series is the simpler one: it orders events by fixed relations of earlier-than and later-than. Once established, these never change — the signing of the Magna Carta is always earlier than the French Revolution, full stop. The A-series is different: it assigns the properties "past," "present," and "future" to events, and these properties genuinely change. Right now this sentence is present; a moment ago it was future; soon it will be past. McTaggart argues that real temporal change requires the A-series — only A-properties change, whereas B-relations are eternal and static.
Here is the problem McTaggart identified: the A-properties are mutually incompatible (nothing can be past, present, and future simultaneously), yet every event must possess all three at some point in its career. The natural reply is that an event is future at earlier times, present at a certain time, and past at later times. But this reply re-uses temporal notions ("at earlier times") to explain the A-series — meaning we need another A-series to explain the first one, generating an infinite regress. McTaggart took this as proof that the A-series is incoherent, and since the B-series alone cannot account for change, he concluded that time is unreal.
Most philosophers reject McTaggart's conclusion while taking his argument seriously. A-theorists (presentists, growing-block theorists) try to defend a coherent account of temporal passage and privileged present. B-theorists accept the static four-dimensional block universe — all times are equally real, and "now" is like "here," a perspective-relative indexical rather than an objective property of the universe. The block universe is sometimes felt to conflict with our experience of time flowing, but B-theorists argue this experience is a cognitive artifact, not a metaphysical fact.
A live controversy is whether special relativity settles this debate. Relativity shows that simultaneity is frame-relative — there is no single "now" that carves the universe at its joints — which seems to favor the block universe. A-theorists respond that relativity constrains the physics but does not by itself determine the metaphysics of temporal passage. The debate remains open, making the philosophy of time one of the few areas where metaphysics, physics, and phenomenology must all be brought to bear simultaneously.
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