|1. Lewis's Terminology: Persistence, Endurance, Perdurance|
Lewis offers a helpful terminology for talking about this issue.
Persistence: a generic, neutral term for the way things exist through periods of time.
Endurance: the view that all of an object is present at every time at which it exists. Objects persist "by being wholly present at more than one time."
Perdurance: the opposed view that only part of something is present at any time at which the thing exists, and persistence is a matter of an object having different stages at different times.
An odd thing: I'd have been inclined to say that the difference between these views is only verbal. The defender of endurance uses "all of" to mean "all spatial parts of" while the defender of perdurance uses "all of" to include all temporal stages as well as all spatial parts. It seems as though we could consistently decide to talk either way (although I think it's a lot easier to think clearly about issues of persistence if we go with perdurance instead of endurance).
But many philosophers, including Lewis and Geach, are convinced that there is a real issue here, and argue for one or the other position.
|2. Arguments for Perdurance (and the 4D view)|
Geach rightly points out that some argments for perdurance are bad arguments. (But I'm not sure who, if anyone, actually offers these bad arguments!)
1. Argument from graphs. We can construct graphs or diagrams in which space and time are represented by axes. Therefore time is a dimension just like space.
Geach is right, this is a terrible argument. Compare: We can construct graphs in which height and weight are represented by axes. Therefore height and weight are dimensions.
2. Argument from logic. We need 4D objects in order to use symbolic logic to represent arguments involving time.
Again, not such a good argument. First, it's not clear that it's true. Adding an argument place for times to every predicate seems just as good. Second, even if it were true, it might just show that there are perfectly good arguments that classical first-order logic is not equipped for. But we knew that anyway! Compare: It's necessary that P. Therefore, it is not possible that not-P. Valid argument, but can't be represented in first-order logic. That's why we develop extensions of first-order logic such as modal logic.
3. Argument from special relativity. I'm inclined to give this one a lot of weight. In relativity theory, how four-dimensional spacetime is divided into space and time is relative to a frame of reference. There is no absolute fact about which events are simultaneous with one another: two events can be simultaneous for one observer but not for another. And there is no "correct" frame of reference.
Most responses to this argument seem mainly to involve hoping that relativity theory will turn out not to be true. (Sometimes expressed with great confidence wit what strike me as absurd territorial claims, such as Geach's confident assertion that issues about time are a matter of logic, not physics. Reminds me of the days when philosophers thought that cosmology was a philosophical, not scientific issue.)
4. Argument from temporary intrinsics. Lewis's argument. I can have contradictory intrinsic properties. (For example, I'm bent over at one time, and straight at another time.) These properties are intrinsic, i.e. determined entirely by the thing that has them. How can I have two properties, both determined entirely by me, and yet which contradict each other?
1. I have a mass of n kilograms (at one time) and I don't have the
property of having a mass of n kilograms (at a different time).
2. Mass is an intrinsic property.
3. My mass is a property of all of me.
These three theses lead to a contradiction. We need to give up at least one. Lewis argues that it's not credible to give up 1 or 2, so we have to give up 3.
Zimmerman, on the contrary, gives up 1 by adopting presentism: the
doctrine that the only facts are facts about the present. (Wow!)
|3. Arguments against Perdurance (i.e. for Endurance)|
1. Argument from change. Geach accepts MacTaggart's view that if the 4D view is correct, there is no change. (Oddly, though, he does this without discussing or apparently accepting MacTaggart's weird definition of what real change would be! MacTaggart things that things can't change on any view of time, so that the only real change is the change events undergo when they go from being future, to being present, to being past.)
But Geach offers no reason whatsoever for thinking this! He simply explains the 4D account of change (different temporal parts of something having different properties), and then insists that that's not really change! (Well, I say it is! So there!) I don't get it. The 4D analysis of what change is seems entirely coherent to me. Since Geach gives no reason for thinking it incorrect beyond mere assertion, he hasn't given me any reason to change my mind.
2. Argument from predication. Geach also argues that many of the things we predicate of people or things can't be predicated of slices. For example, he says that a slice can't be a philosopher, and a slice can't eat mice.
Well, why not? It's probably true that a momentary slice can't do these things (you have to have enough time to think philosophical thoughts, or to chew on the mouse), but I don't see any problem about longer slices doing such things.
3. Critique of hybrid view. Geach spends quite a while criticizing an odd hybrid view: that the physical world is a 4D object, while the mind moves through it. He's right that this is a crazy view. But since no one who accepts the 4D view holds it, it can't count against the 4D view! (Geach notes that (a) the view requires an extreme dualism of mind and body, and (b) it seems to lead to a kind of fatalism, to the view that the mind can't do anything to the physical world except observe it. I think it's often true that the cause of worries about the possibility of free will are attached to some form of dualism. Once we see the mind as part of the world, not some sort of nonphysical entity separate from it, I think a lot of worries about free will are dissolved. But that's a topic for a different day.)