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Presentism and eternalism are competing answers to the question of temporal ontology: what exists in time? Presentism holds that only present entities exist — past objects like dinosaurs and future objects like Mars colonies have no being whatsoever. Eternalism (or four-dimensionalism) holds that all times are equally real: past, present, and future entities all exist, and the apparent specialness of the present is a feature of our perspective, not of reality. The growing block theory occupies a middle ground, holding that the past and present exist but the future does not yet. These positions connect tightly to the A-theory/B-theory distinction: presentism naturally pairs with A-theory (the present is metaphysically privileged), while eternalism fits B-theory (all times are ontologically on par). The debate has consequences for the truthmakers of past-tense statements, the reality of temporal passage, and compatibility with special relativity.
Read Sider's Four-Dimensionalism chapter 2 for the eternalist case and Markosian's 'A Defense of Presentism' for the opposing view. Focus on the cross-temporal relations problem: how can a presentist account for the truth of 'Caesar was murdered' if Caesar does not exist?
The question of temporal ontology — what exists in time — touches something viscerally puzzling: are Julius Caesar and the year 2150 as real as you are right now? Presentism says no: only present entities and events exist. Caesar has ceased to exist entirely; the year 2150 does not yet exist. Eternalism says yes: the past, present, and future are all equally real, arranged in a four-dimensional spacetime manifold. What distinguishes them is their temporal location, not their degree of reality — just as distant spatial locations are real even though you're not there. Building on your understanding of A-theory and B-theory, you can see the natural alignment: presentism pairs with A-theory (the present has special ontological status), while eternalism pairs with B-theory (all times are ontologically on par).
The growing block theory occupies an intermediate position: the past and present have "grown" into existence and are equally real, but the future does not yet exist. This captures the intuition that the past is fixed and settled while the future is genuinely open. It avoids the eternalist claim that future events are already "there" in the block, while avoiding the presentist's complete elimination of the past. The growing block has its own problems — it implies there is an objective boundary between the real past-present and the unreal future, which faces similar difficulties to the presentist's privileged present when confronted with special relativity.
The most powerful objection to presentism comes from special relativity. Relativity eliminates absolute simultaneity: whether two spatially separated events are simultaneous is frame-relative. If the present is defined as all simultaneous events, then "the present" is also frame-relative — there is no single objective present slice of spacetime. This seems to undermine presentism's core claim that only the present exists: *whose* present? Presentists respond in several ways: some relativize the present to a reference frame; others invoke a neo-Newtonian spacetime that restores absolute simultaneity at a different level; others argue that relativity doesn't address ontological questions directly.
Within metaphysics (independent of physics), the sharpest challenge to presentism is the problem of cross-temporal relations and truthmakers. "Caesar was murdered" seems true. But if Caesar doesn't currently exist (on presentism), what makes it true? There is no Caesar in the present inventory to serve as a constituent of any fact. Presentists propose various solutions: ersatz past times (abstract objects representing how things were), primitive "was" operators that don't require past entities to exist, or haecceities (abstract individual essences that persist even when the individual doesn't). Each solution carries ontological commitments. Eternalists face no such problem — Caesar exists at his temporal location, and tensed statements are made true by facts at those locations. This asymmetry is one of the strongest arguments for eternalism, and understanding it requires holding together the metaphysical framework from A/B theory, the truthmaker questions from ontology, and the physics of spacetime.
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