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Three-dimensionalism holds that ordinary objects are three-dimensional entities wholly present at each moment they exist, changing by having different properties at different times. A person is wholly present at age 10 and wholly present at age 20, enduring through time without temporal parts, with change accounted for by temporal property variation rather than different parts at different times.
Contrast three-dimensionalism directly with four-dimensionalism (its main competitor). Ask: if you are wholly present now and wholly present five years ago, how can you have different properties at both times — aren't you both straight and bent at once? How three-dimensionalists answer this question reveals the core philosophical work the view must do.
From your work on philosophy of time and persistence and change, you know the central puzzle: how can a single object have incompatible properties? A banana is green on Monday and yellow on Friday. These seem to contradict each other — the same banana can't be both green and yellow. The endurance or three-dimensionalist answer is that "green" and "yellow" are not bare intrinsic properties but are relativized to times: the banana is green-at-Monday and yellow-at-Friday. There's no contradiction because the property ascriptions are indexed to different times, just as "tall for a five-year-old" and "short for an adult" are indexed to different comparison classes.
The core commitment of three-dimensionalism is whole presence: an object is *entirely* present at each moment of its existence — not merely a temporal stage or cross-section, but the full object. When you see a person today and saw them five years ago, you saw the same entity on both occasions, not different temporal parts of a four-dimensional entity. This matches common-sense ontology: we ordinarily think of persisting objects as things that *go through* time, not things that are *extended in* time the way they are extended in space.
The main problem three-dimensionalism must solve is the problem of temporary intrinsics (Lewis's challenge). If you are wholly present at two times and have different shapes at those times, it seems your shape is not a genuine intrinsic property — it must be relational, indexed to a time or a temporal counterpart. But shape seems paradigmatically intrinsic: your shape should be determined by how you are in yourself, not by your relation to a time. Three-dimensionalists respond in several ways: they can relativize properties to times (properties are time-indexed predicates like "bent-at-t"), appeal to adverbialism (you have properties *temporally* rather than having time-indexed versions of them), or treat temporal instantiation as primitive. Each response preserves whole-presence but at some cost to how we understand intrinsic properties.
The contrast with four-dimensionalism (your builds-toward topic) sharpens the stakes. The four-dimensionalist says you are not wholly present at each time — you have distinct temporal parts at different times, and your "change" is just different temporal parts having different properties, the way your head and feet can have different temperatures at the same time. Four-dimensionalism dissolves the problem of temporary intrinsics but may conflict with ordinary thinking about identity over time. Three-dimensionalism preserves intuitive identity at the cost of a more complex theory of property instantiation. The debate ultimately connects to philosophy of time: if the A-theory is correct and the present is metaphysically special, three-dimensionalism may be the more natural fit; if the B-theory treats all times as ontologically equal, four-dimensionalism fits more naturally into an eternalist block universe.
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