The Trouble with 4D Objects
1. A 3D enduring object is wholly present to us. But a 4D object which is extended also in the temporal dimension is not wholly present; only its time-slice right now is present to us. Its other temporal parts are nowhere to be seen. Hence it entails a controversial claim that presentism — the claim that only present objects exist — is false.
A related problem is a practical one. What can we do with a 4D thing? Its past is gone; its future is not yet, all we have is its present temporal slice. Of what use is postulating inaccessible to any human action parts? Few people think possible worlds to be more than conceptual devices, but four-dimensionists are hard-core realists about the past: they think it exists right now as a part of any 4D entity.
2. A 3D object can stay the same; it may be able to endure. But a 4D object does not actually perdure and stay the same; it grows with the passage of time. Friends of temporal parts say things like “a statue exists from 10:00 until 11:00″ (rather than at 10:00 and at 11:00). But a statue considered at 10:30 does not exist at 11:00. It keeps accumulating part after part every second. So, what causes this growth? How is ticking of a clock able to alter an innocent object so drastically?
In other words, a 4D object perdures only in its spatial aspect; as time goes on it changes. How counter-intuitive is that? A 3D theorist can, on the contrary, say, that the same object X has been persisting in being from 10 to 11 o’clock without having to make the strange claim that X has grown from an embryo into a fully actualized 4D worm just by sitting there and without consuming any food suitable for such creatures.