The A-Theory and the B-Theory

McTaggart's argument bequeathed the philosophy of time its central division. Every theory of time must say whether the tensed facts — what is past, present, or future — are fundamental features of reality, or whether the only objective temporal facts are the tenseless relations of earlier than and later than. The first answer is the A-theory (the tensed, or dynamic, view); the second is the B-theory (the tenseless, or static, view). The dispute is not about the physics of clocks but about metaphysics and language: is there an objective, moving now, and does time genuinely pass — or is "now," like "here," merely a marker of the speaker's location in a reality where all times are equally real and nothing flows?

This page develops the two theories, the semantic debate that has become their main battleground, and the arguments on each side; the ontological versions (which times exist) are on the presentism/eternalism page, and the question of passage on the passage and experience page.

The two theories

• The A-theory (tensed / dynamic). Tense is objective and irreducible. There is a privileged, unique present, and events genuinely change their A-properties: what is now future will become present and then recede into the past. Reality itself is dynamic — the sum of what is present (or of what has happened) is continually updated. Passage is a real feature of the world, not a feature of our viewpoint. Presentism, the growing block, and the moving spotlight are all A-theories.
• The B-theory (tenseless / static). The only objective temporal facts are B-relations: event $e_1$ is earlier than $e_2$, and that fact never changes. "Past," "present," and "future" are indexical — they express the temporal location of the utterance, exactly as "here" and "there" express spatial location. No time is metaphysically privileged; the universe is a four-dimensional block in which all events tenselessly exist, and the appearance of passage is to be explained, not built into the ontology.

A useful way to see the contrast: for the B-theorist, "it is now 2026" is true when uttered in 2026 and false when uttered in 2025 — it is like "I am here," true wherever said. For the A-theorist, there is a further, absolute fact — that 2026 is present — which is not merely a fact about when the sentence is spoken.

The semantic battleground: the old and new B-theory

The twentieth-century debate turned decisively on language — specifically, on whether tensed sentences can be translated into, or given truth conditions by, tenseless ones.

The old (translation) B-theory. Early B-theorists (Russell, and the logical tradition) held that tensed sentences could be translated without loss into tenseless ones. "The meeting is now occurring," said at $t$, means the same as the tenseless "The meeting occurs (tenselessly) at $t$." If the translation succeeds, tense is eliminable and carries no ontological weight.

The A-theorist's reply (Prior). A. N. Prior's "Thank Goodness That's Over" objection sank the translation programme. When, at the end of a painful experience, I sincerely say "Thank goodness that's over," I am not expressing relief that the experience is earlier than this utterance (a tenseless B-fact I could contemplate with indifference), nor that it ends at some date $t$ (I may not know the date, and would feel no relief on being told "your pain is tenselessly located at 3:00"). What I am glad about is precisely that the pain is past — now over. The tensed fact is not captured by any tenseless paraphrase; something is lost in translation. Prior concluded that tense is ineliminable and real.

The new (token-reflexive / date) B-theory. B-theorists retreated to a stronger position, conceding that tensed sentences cannot be translated into tenseless ones but denying that this shows tense is ontologically real. The move: separate the meaning of a tensed sentence from its truth conditions. A tensed sentence-token is true in virtue of tenseless facts — the token "the meeting is now" is true iff the meeting occurs (tenselessly) simultaneously with that very token (token-reflexive version, Mellor) or at the token's date. So the world contains only tenseless facts; tensed language is an indispensable tool for creatures located in time, but it does not require tensed facts as truthmakers. This "new B-theory of language" (Mellor, Smart) is the mainstream B-theoretic position: tense is real in thought and language, unreal in reality.

Arguments for the A-theory

• The experience of passage. Time's passing is among the most vivid facts of experience; the A-theory takes this at face value, while the B-theory must dismiss it as a systematic illusion. (The B-theorist's reply is the deflationary account of temporal experience.)
• The openness of the future / the special status of the present. Our attitudes are tensed: we dread future pains and are relieved by past ones (Prior). Deliberation seems to presuppose an open future and a moving now. The A-theory grounds these; the B-theory must explain them away as perspectival.
• Becoming. The A-theorist holds that things genuinely come into being — that reality is added to — which the static block seems to deny.

Arguments for the B-theory

• Economy and McTaggart's regress. The B-theory needs only tenseless facts and escapes McTaggart's contradiction (Step 2) cleanly, since it never posits the incompatible A-properties. It analyses change à la Russell — an object has one property at $t_1$ and another at $t_2$ — with no A-series required.
• The argument from relativity. This is the B-theory's heaviest weapon. Special relativity denies any frame-independent present or worldwide "now" (the relativity of simultaneity). But the A-theory needs an objective, global present. If physics has no room for one, so much the worse for the A-theory — the Rietdijk–Putnam argument pushes hard toward the tenseless block universe.
• The problem of the rate of passage. If time "flows," how fast? "One second per second" is trivial or empty, and any non-trivial answer launches a regress. The B-theorist argues that "passage" is not a coherent extra feature at all.

Where this sits

The A/B distinction is the backbone of the temporal question and the direct descendant of McTaggart's two series. It is not yet a theory of what exists — that is the further, ontological choice among presentism, eternalism, the growing block, and the moving spotlight, each of which is a way of being an A- or B-theorist. Its sharpest test is physical: whether an objective present can survive the relativity of simultaneity, which is why the block universe looms so large. And its residual puzzle — whether, and how, time passes — is the subject of the next page.