Stage theory proposes that objects are instantaneous temporal stages and apparent persistence consists of causal connections and resemblance among stages. This eliminates puzzles about identity through time by denying strict identity persistence; different temporal stages are causally and mereologically related without being identical to one another.
Compare stage theory systematically with four-dimensionalism and three-dimensionalism, examining how each framework handles problem cases like fission, fusion, and temporary non-existence.
Thinking stage theory means nothing persists or truly changes. Confusing stage theory with presentism or the growing block theory.
From your study of four-dimensionalism, you know that one major view treats persisting objects as extended through time as well as space — a person, on this view, is a four-dimensional entity whose temporal parts or "stages" are as real as its spatial parts. Stage theory accepts this ontological picture but draws a surprising conclusion about identity: when you point to yourself today, you are not pointing to the whole four-dimensional worm. You are pointing to a stage — an instantaneous or very brief temporal slice — and that stage is the person.
This might seem like a minor terminological dispute, but it has real metaphysical consequences. On the standard four-dimensionalist view (the "worm theory"), strict identity holds *across* time: you today and you yesterday are the *same entity* — they are both parts of the extended four-dimensional worm that constitutes you. Stage theory denies this. The stage that is you today is numerically distinct from the stage that was you yesterday. They are causally related, they resemble each other, they are part of the same career or worm in a loose sense — but they are not identical. What we ordinarily call "persisting through time" is really a sequence of distinct but connected stages, not a single entity enduring intact.
Why accept this seemingly counterintuitive view? Stage theory was developed by Theodore Sider and others partly to handle puzzles about temporary properties. Consider that you were once sitting and are now standing. On the worm view, the same object (you) has contradictory properties: being-sitting and being-standing. The standard response is to index these to times: you are sitting-at-t₁ and standing-at-t₂. Stage theory handles this differently: the stage at t₁ simply is sitting, full stop, and the stage at t₂ is standing, full stop. There is no contradiction because the stages are numerically distinct objects. This makes property attribution clean and avoids any reliance on temporal indexing.
The more dramatic payoff comes with cases like fission: a person is split (imagine a Star Trek transporter creating two copies). Stage theory can say: before the split, there is one stage; after the split, there are two distinct continuation-series, each related to the prior stage by the right causal and resemblance connections. No violation of identity occurs because identity was never what connected the stages in the first place. The connection is a counterpart relation — a matter of fitting into the same temporal career — not strict identity. This elegantly dissolves fission cases that bedevil identity-based theories of personal identity, though it requires accepting that ordinary self-reference picks out a stage rather than an enduring thing, and that "I will exist tomorrow" is a claim about a future stage in my continuation-series rather than a claim about my own future existence.
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