A-theory of time

Definition and Core Claims

In contemporary metaphysics, A-theory of time (or simply A-theory) designates any view according to which tensed facts—facts about what was, is, and will be—are fundamental features of reality. A-theorists typically maintain that:

• There is an objective present, a metaphysically privileged “now.”
• Temporal passage or becoming is real: events come into being, are present, and then pass away into the past.
• Tensed statements (e.g., “It is now raining”) cannot be fully captured by tenseless descriptions (e.g., “It rains at t at location x”).

The A-theory stands in contrast to the B-theory of time, which treats time as an ordered series of events related by earlier-than and later-than relations without an objectively distinguished present. For B-theorists, all times are equally real, and talk of past, present, and future is either indexical or reducible to tenseless relations.

A-theory is therefore characterized by its commitment to irreducible tense and real temporal becoming, often expressed through metaphors of a “moving present” or a “growing” reality.

Historical Background

The terminology originates in J. M. E. McTaggart’s 1908 article The Unreality of Time. McTaggart distinguished:

• The A-series: events ordered as past–present–future, where each event changes in its A-properties (was future, becomes present, then becomes past).
• The B-series: events ordered as earlier–than / later–than relations, which do not change.

McTaggart argued that genuine time requires the A-series, but that the A-series is contradictory: each event would have to be past, present, and future, which he regarded as impossible. He concluded that time is unreal. Later philosophers accepted his distinction but rejected his skepticism, developing positions that embraced either the A-series (A-theorists) or the B-series (B-theorists) as fundamental.

In the mid-20th century, Arthur N. Prior revitalized the A-theory through his development of tense logic, a formal system designed to capture the logical structure of tensed discourse. Prior argued that tensed language reflects objective features of reality rather than mere linguistic convenience. His work provided A-theorists with a sophisticated logical framework and helped distinguish A-theoretic metaphysics from mere psychological impressions of temporal flow.

Subsequent analytic metaphysics consolidated the terminology: A-theory came to mean any theory that affirms the metaphysical reality of tense and temporal becoming, while B-theory named approaches that deny these as fundamental and seek tenseless analyses.

Main Variants of the A-Theory

Under the A-theory umbrella, several distinct positions are commonly distinguished. They differ mainly in how they understand the ontology of past, present, and future.

1. Presentism

   • Core claim: Only present objects and events exist (in the most robust sense). The past has existed; the future will exist; but only what is present is real now.
   • Reality is thus “thin,” containing only the instantaneous present.
   • Tensed truths about the past (“Socrates taught in Athens”) are usually grounded in present traces (memories, records) or primitive past-directed facts.

   Presentism is perhaps the most intuitive A-theoretic view and aligns with ordinary language and many religious or theological perspectives. Critics question how it can truthfully accommodate claims about non-present entities and whether it can be reconciled with modern physics.

2. The Growing Block Theory

   • Core claim: The past and present exist; the future does not.
   • Reality is like a “growing block”: with the passage of time, new “slices” of reality are added at the front edge (the present), which then become part of an ever-expanding past.
   • Temporal passage consists in the growth of this block, with the present as the leading edge.

   Supporters find this attractive because it preserves the reality of the past while maintaining a robust sense in which the future is open. Debates focus on how to characterize the “edge” of the block and what makes that edge objectively present.

3. The Moving Spotlight Theory

   • Core claim: Past, present, and future events all exist (like the B-theorist’s “block universe”), but one time is objectively present in virtue of being “illuminated” by a metaphorical moving spotlight.
   • Temporal passage is the changing fact of which temporal location is privileged.

   This view preserves both the eternal existence of all times and a robust A-theoretic present. Critics charge that it risks redundancy: if all times equally exist, adding a moving spotlight may appear metaphysically superfluous or obscure.

4. Hybrid and Neo-A-Theoretic Approaches

   Some contemporary philosophers propose hybrid views that combine B-theoretic structures with A-theoretic primitives, or that treat tense as fundamental at a more “high-level” description of the world but derivable from deeper tenseless physics. Others distinguish between:

   • Dynamic A-theories, emphasizing genuine creation or coming-into-being.
   • Static A-theories, where the privilege of the present is fundamental but does not involve ontological growth.

These variants illustrate that A-theory is not a single doctrine but a family of positions unified by commitment to objective tense and real becoming, while differing on which times exist and how passage is best characterized.

Contemporary Debates and Criticisms

Modern discussions of the A-theory focus on its coherence, its relation to science, and its explanatory power relative to rival accounts.

1. McTaggart’s Paradox and Coherence

   Critics inspired by McTaggart argue that attributing changing A-properties to events (future → present → past) leads to contradiction or infinite regress: an event would have to be past, present, and future, or possess these properties at different times, generating new tensed levels ad infinitum.

   A-theorists respond by:

   • Distinguishing levels of predication (e.g., “event e was future, is present, will be past”) to avoid attributing incompatible properties simultaneously.
   • Treating tensed facts as primitive, not reducible to combinations of tenseless claims, thereby stopping the regress.

2. Compatibility with Relativity and Physics

   Einstein’s special relativity undermines the idea of an absolute, observer-independent global present by showing simultaneity to be frame-relative. This has led many physicists and philosophers to favor B-theoretic or “block universe” interpretations.

   A-theorists propose several lines of response:

   • Some attempt to reconcile A-theory with relativity by positing a hidden or privileged foliation of spacetime not yet detected empirically.
   • Others argue that physics describes only part of reality’s structure, leaving room for a metaphysically privileged present that does not show up directly in physical equations.
   • A different strategy interprets relativity instrumentally or as incomplete, suggesting future physics might restore a more A-theoretic picture.

   Critics contend that these moves either conflict with well-confirmed physics or add untestable structure.

3. Explanatory Power and Experience of Time

   Proponents of A-theory argue that it best captures:

   • Our phenomenology of time: the felt “flow” of time and the asymmetry between past and future.
   • The apparent openness of the future and the significance of decision, anticipation, and regret.
   • The directionality of certain processes (e.g., causal influence, thermodynamic increase of entropy), which seem to presuppose becoming.

   B-theorists and other critics reply that:

   • Phenomenological impressions can be explained in psychological or cognitive terms without positing objective passage.
   • Asymmetries in causation and entropy may be grounded in boundary conditions and physical laws, not in A-theoretic becoming.
   • The A-theory may introduce extra metaphysical structure without clear explanatory gain.

4. Tense, Truth, and Language

   Another focus is whether tensed language indicates an A-theoretic reality or can be systematically paraphrased into tenseless terms. B-theorists argue for tenseless truth conditions (e.g., “It is raining” is true iff it rains at time t simultaneous with the utterance). A-theorists maintain that such paraphrases either fail to capture the original meaning or presuppose tense at some stage.

   This has led to detailed debates in the philosophy of language and logic about:

   • The semantics of indexicals and temporal operators.
   • Whether tense is an ineliminable aspect of truth-conditions.
   • The role of tense logic in representing metaphysical commitments about time.

Overall, the A-theory of time remains a central, vigorously contested position in the metaphysics of time. It continues to shape discussions about reality’s temporal structure, the nature of change and becoming, and the relationship between everyday temporal experience and the highly formal picture offered by contemporary physics.