The Nature of Time

Of the three questions that organise this section, the temporal one is the richest and the most contested: granting that there is time, is its passage real? Does the world contain an objective, moving present that divides a fixed past from an open future — or is the division into past, present, and future merely a feature of our perspective on a reality in which all times are, tenselessly, on a par? The question is not whether time exists but what its structure is: whether tense is a fundamental feature of reality or a projection, and whether "time flows" states a deep truth or a seductive metaphor.

This page maps the terrain and states how the pieces fit; the time subfolder develops each in depth — McTaggart's argument, the A- and B-theories, presentism and eternalism, passage and experience, the arrow of time, persistence, and time travel.

Two axes: the A/B distinction and passage

The modern debate descends from J. M. E. McTaggart's 1908 argument for the unreality of time, whose lasting contribution was a pair of distinctions. Any moment or event can be described in two ways:

• By its A-series properties — being past, present, or future. These are monadic, tensed, and changing: a given event is first future, then present, then ever more past.
• By its B-series relations — being earlier than, simultaneous with, or later than other events. These are dyadic, tenseless, and permanent: if the Battle of Hastings is earlier than the Moon landing, it always was and always will be.

McTaggart argued that genuine change requires the A-series (the B-series is frozen), yet the A-series is contradictory (every event must be past, present, and future, which are incompatible), and concluded that time is unreal. Few accept the conclusion, but the framing endures and names the central division:

• The A-theory (tensed view) holds that the A-series is fundamental: there is an objective present, and the tensed facts — what is happening now — are irreducible features of reality. Passage is real.
• The B-theory (tenseless view) holds that the B-series is fundamental: reality consists of events in earlier–later relations, and "past," "present," and "future" are indexical, like "here" and "there," marking only the speaker's temporal location. Passage is an appearance to be explained away.

These are developed on the A-theory/B-theory page, along with the semantic debate over whether tensed sentences ("the meeting is now") have tenseless truth conditions.

The ontological question: what times exist?

Cutting across the A/B distinction is a question about ontology — which times, and which objects at which times, exist? Four positions dominate (developed on the presentism/eternalism page):

Presentism and the growing block are naturally A-theoretic (they build in a privileged present); eternalism is the natural home of the B-theory and of the four-dimensional block universe. The moving spotlight is the curious hybrid that grafts a moving A-series present onto an eternalist block.

Each position pays a price. Presentism struggles to supply truthmakers for truths about the past ("there were dinosaurs") when the past does not exist, and to accommodate cross-temporal relations. Eternalism is charged with losing the passage of time and making our sense of "now" an illusion. The growing block faces the "how do you know you are at the growing edge?" (the dead past) objection. These are weighed on their pages.

The great external constraint: relativity

The temporal debate is the place where physics presses hardest on metaphysics. Special relativity abolishes absolute simultaneity: whether two spatially separated events happen "at the same time" depends on the inertial frame — the relativity of simultaneity. But presentism and the growing block need a global, frame-independent present — a worldwide "now" that is ontologically privileged. If there is no such thing, these views appear to be in serious trouble.

This is the Rietdijk–Putnam–Penrose argument: relativity has no place for an objective present, so eternalism and the block universe are the natural, perhaps forced, ontology. A-theorists reply by relativising existence to frames, retreating to a point-present, or adopting a neo-Lorentzian interpretation with a real but empirically hidden preferred foliation. The dispute is unresolved, and it is the clearest case in the book of a metaphysical question shaped by a physical result.

Passage, direction, and persistence

Three further problems flow from the two axes:

• Passage. Even granting an objective present, does time flow, and if so how fast? "One second per second" looks empty, and the demand for a rate threatens a regress. The B-theorist offers a deflationary account of why time merely seems to pass — the subject of passage and experience, which draws on the phenomenology of the "specious present."
• Direction. Time has an arrow: we remember the past not the future, causes precede effects, entropy increases. Is this asymmetry built into time itself, or grounded in the asymmetric distribution of its contents — above all the thermodynamic gradient traced to a low-entropy past? See the arrow of time and its physical basis in the thermodynamic arrow.
• Persistence. How does a single object exist through time — by being wholly present at each moment (endurantism) or by having distinct temporal parts at each (perdurantism / four-dimensionalism)? The answer is entangled with eternalism and the block universe: four-dimensionalism sits naturally with Minkowski spacetime. See persistence.

And at the limit, the coherence of the whole framework is tested by time travel: closed timelike curves are permitted by general relativity, and the grandfather paradox probes what a consistent past-directed journey would require.

Where this sits

The nature of time is the temporal question of the three that structure this section, and the one where the ontological and geometric questions most visibly converge — for the block universe is at once a thesis about time (eternalism), about spacetime ontology (a four-dimensional whole), and about geometry (the Minkowski structure of special relativity). It also reaches beyond physics: the openness of the future bears on free will and determinism, and the beginning of time on cosmology and the philosophy of religion. The next pages take the strands in turn, beginning with the argument that started the modern debate — McTaggart on the unreality of time.