Consider a simple question: when you look at yourself in a childhood photograph, what exactly are you seeing? One natural answer is that you're seeing yourself—the very same entity that now reads these words. But here's the puzzle: that child had different properties than you do now. Different height, different beliefs, perhaps different eye color.

How can one and the same thing be both three feet tall and five feet tall? This isn't a contradiction if the properties are had at different times. But how exactly does that work? The answer divides contemporary metaphysics into two fundamentally different pictures of what objects are and how they persist through time.

Four-dimensionalism, or perdurantism, offers a radical proposal: you are not wholly present right now. Rather, you are spread out through time like a worm, and what exists at this moment is merely one temporal part of you—a stage in your four-dimensional career. This view transforms our understanding of objects, change, and time itself.

The perdurantist thesis can be stated precisely: ordinary objects persist through time by having distinct temporal parts at each moment of their existence. Just as a road extends through space by having different spatial parts in different locations, you extend through time by having different temporal parts at different moments.

This isn't merely an analogy. Four-dimensionalists take the parallel seriously. Consider how we understand spatially extended objects. A flagpole has a top part and a bottom part. Neither part is the flagpole; rather, the flagpole is the mereological sum of these parts. Similarly, your childhood stage and your current stage are both parts of the temporally extended entity that is you.

The view gains considerable support from the theory of special relativity. Relativity reveals that the distinction between space and time is frame-relative—what counts as purely spatial extent versus temporal duration depends on one's reference frame. This suggests that spacetime is the fundamental arena, and the space-time distinction is less fundamental than we pre-theoretically suppose. If so, treating objects as four-dimensional spacetime worms seems natural.

David Lewis developed the most systematic perdurantist framework. On his view, ordinary predications involving time should be analyzed as relations to temporal parts. When we say 'Alice was short,' we mean that some past temporal part of Alice instantiates shortness. The property is had simpliciter by the part, not by Alice-as-a-whole in some modified, relativized way.

Takeaway

Objects may persist like roads extend—not by being wholly present everywhere along their length, but by having different parts at different locations in time.

Endurantism maintains that objects are three-dimensional entities wholly present at each moment of their existence. When you existed yesterday and exist today, the whole of you was there yesterday and the whole of you is here today. There are no temporal parts; rather, you persist by enduring—by being multiply located in time.

This view captures a powerful intuition. When I anticipate tomorrow's dental appointment, I am not concerned for some future temporal part of me. I am concerned for myself—the very entity now dreading the procedure. Self-interest, moral responsibility, and personal identity seem to presuppose that we are wholly present at different times, not that different parts of us exist at different times.

The endurantist faces an immediate technical question: how should we understand property-possession across time? If I am wholly present both when I am sitting and when I am standing, do I have contradictory properties? The standard endurantist response relativizes property-possession to times. I have the property of sitting-at-t₁ and standing-at-t₂. These are not contradictory because they are different properties.

Some endurantists prefer adverbialism: I have the property of sitting t₁-ly and standing t₂-ly. Properties are had in temporally modified ways. Others invoke time-indexed relations to properties rather than modified instantiation. Each strategy attempts to preserve the intuition that objects are wholly present while explaining how they can be different at different times.

Takeaway

The endurantist wager is that our ordinary self-conception—as entities wholly present across time rather than scattered through it—tracks something metaphysically fundamental.

Lewis argued that the problem of temporary intrinsics provides a compelling reason to prefer perdurantism. The puzzle is this: objects change their intrinsic properties over time. A banana is first green, then yellow. Being green and being yellow are intrinsic properties—they characterize the banana in itself, not in relation to anything else.

But if the banana is wholly present when green and wholly present when yellow, and if it has these properties intrinsically (not relative to times or anything else), then it would seem the banana is both green and yellow simpliciter—a contradiction. Lewis concluded that either greenness and yellowness are not genuinely intrinsic, or the banana is not wholly present at each time.

The perdurantist solution is elegant. The banana has a Monday-part that is green simpliciter and a Friday-part that is yellow simpliciter. There is no contradiction because no single entity has both colors. The temporary intrinsics problem dissolves once we recognize that change involves the replacement of temporal parts rather than the transformation of an enduring whole.

Endurantists have developed sophisticated responses. The relationalist strategy holds that being green is not intrinsic but involves a relation to a time—the banana bears the greenness relation to Monday. Others accept that properties are had relative to times, denying that this makes them non-intrinsic. Some philosophers question whether the intrinsic-relational distinction is as sharp as Lewis supposed. The debate remains unresolved, but it illustrates how fundamental questions about persistence connect to equally fundamental questions about properties and their instantiation.

Takeaway

How objects change reveals something about what objects are—whether they are subjects that transform or sequences of stages that differ.

The debate between perdurantism and endurantism is not merely about objects and their parts. It reflects deeper choices about the nature of time, the structure of properties, and what it means for anything to persist through change.

Perdurantism offers systematic elegance: it unifies space and time, dissolves the temporary intrinsics puzzle, and aligns with relativistic physics. Endurantism preserves intuitions about identity, self-concern, and what it means to be the same thing over time.

Perhaps most striking is what both views agree on: the question of how objects persist cannot be answered without taking a stand on the fundamental architecture of reality itself. The nature of ordinary things, it turns out, is anything but ordinary.