Temporal Parts (Four-Dimensionalism)

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This is the 59th post in my Theories of Knowledge and Reality series. The last post looked at the final view of what I consider the standard accounts of personal identity (the dualist, psychological, bodily, and brain views). As I said at the end of the last post, it's pretty common for people to look at all the difficulties raised against those views and then opt for something more unconventional. The next few posts will look at three such unconventional views: temporal parts (or four-dimensionalism), conventionalism, and nihilism.

The temporal parts view is sometimes called four-dimensionalism, because it takes persisting objects to be spread out across time rather than being wholly present at a time. The three-dimensionalist view takes me to be fully present at every moment of my existence. It's not as if there are parts of me at past and future times, with just some small part of me here right now. I'm fully present at this moment. I was fully present at each past moment of my existence, and I will be fully present at each future moment of my existence. Three-dimensionalists call this kind of persistence through time endurance.

Four-dimensionalism, on the other hand, takes us to be spread out across time, and at each moment it's only the part of me at that time that's present. My current temporal part is here now, and I have past and future temporal parts. All my spatial parts are here right now (at least all the ones that parts of me at this time). But I have temporal parts in the past and future. The whole object across time that is me is called a space-time worm (taking the analogy from a worm composed of segments as its spatial parts). The shorter segments are called stages of that worm, with the smallest stages perhaps being instantaneous stages, which would be infinite in number if time is infinitely divisible. Four-dimensionalists call this kind of persistence through time perdurance.

Although it might not be the most intuitive view to think of me as spread out across time, we do think four-dimensionally about some things, in particular events. Take an event like a baseball game. It is composed of temporal parts. We call them innings. Each inning has two temporal parts (except sometimes the last inning of the game), the top and bottom. A presidential race is an event that is composed of several phases, the primary stage, the general election campaign, and the election day itself. Each of those sub-events is a stage of the entire four-dimensional event that we call an election season. We could say the same thing of any event, such as the War of 1812, the Reformation, an episode of your favorite TV show, or my composing of this post for my blog. Each event has parts that occur for part of the period of time that the event is going on. The four-dimensionalist is just saying that we are also composed of temporal parts in a similar way. There is a part of me that corresponds to the event of my pre-natal existence. Another part of me corresponds to the event of my time in middle school. Another, longer, part of me corresponds to my entire childhood. Another part of me is the instantaneous stage of me at the very instant this post will be online. Another part of me corresponds to my entire adulthood (much of which, I expect, has not occurred yet).

One of the strongest arguments for four-dimensionalism is that it can so easily handle a lot of the problem cases for persistence across time. Take the splitting cases from previous posts. Lieutenant William Riker undergoes a transporter accident and ends up rematerializing both on the planet (as the man who later comes to be called Lieutenant Tom Riker) and on the ship (as the man who later gets promoted to become Commander Will Riker). With three-dimensionalism, you can say that the original becomes one of the two future guys, but the other one is not the original (but it seems arbitrary to pick one over the other), or you can say that the original dies in this case (which is odd if you think either one would be the original without the existence of the other). What you can't say is that both are the original, unless you insist that they are the same guy, who now has two sets of experiences and hates himself, but he's fully present hating himself and fully present being hated by himself, and the instance of himself who is fully present doing the hating is not the instance of himself who is fully present being hated. You might be able to tell a story to make all that work (I think it takes adopting something unconventional, but I don't think it's necessarily incoherent), but it seems strange to say such odd things just to maintain a picture of enduring people.

The four-dimensionalist can say something much more straightforward. There are two space-time worms. One worm starts with Lt. Riker before the transporter accident and runs through Commander Will Riker. The other starts with Lt. Riker before the accident and runs through Tom Riker. These two worms share all the initial stages, the same way two roads the merge share a stage while they run together while remaining two roads. The shared-stage gut at the outset can truly say that he will become Tom Riker and Commander Will Riker, because at that stage it is true that the stage is related in the right way to both future guys. He is a stage of both worms.

Similar things can be said about split-brain fission cases. To use Ted Sider's example, Ted's two brain hemispheres get split and transplanted into two different bodies. Call one Ed and the other Fred. Just as in the Riker case, the 4Der can say that Fred has the right relation to both Ed and Fred so that he can say that he will be Ed and will be Fred, without Ed being Fred. That's only because Ted is a part of a worm that Ed is a part of while also being part of a worm that Fred is part of, while Ed and Fred are never parts of the same worm.

There are a number of other problem cases that four-dimensionalism can handle very well. A lump of clay becomes a statue and then gets melted down again into clay. Is the lump of clay just the same thing as the statue? Well, the lump has a longer existence, so they can't be the same thing. One 3D approach is to take the lump to compose the statue, but that means two things are coinciding in the same place at the same time, made out of the same stuff. If you don't like coincident but distinct objects, four-dimensionalism can handle the problem. The lump is just a longer object in time, and the statue is a temporal part of it. There is only one instantaneous object there at any stage of its existence. It is always part of the lump-worm, and for part of its existence it's also part of the statue-worm. But never are there two 3D objects in the same place at the same time.

Another case is the cat who loses a tail. Call the cat Tibbles. Presumably an object exists before the tail is amputated that is all of the cat except the tail. Call that Tib. Tib is part of Tibbles. Tibbles has a tail, and that tail is not part of Tib. If the tail comes off, then do Tibbles and Tib merge? But they weren't the same object before. How can they be the same object now? Presumably Tib doesn't go out of existence merely because this extra thing, a tail, is no longer attached to it. But  Tibbles doesn't cease to exist. Tibbles just now doesn't have a tail. So are Tib and Tibbles two objects in the same place at the same time? Or should we deny that Tibbles has a part that is all of it but a tail? None of these options seems entirely satisfactory. But with temporal parts, the problem goes away easily. There's a Tibbles-worm and a Tib-worm. The Tibbles-worm is the whole cat across time, which has a tail and then doesn't. The Tib-worm is the cat-minus-the-tail before the amputation and the cat afterward. The worms merge. Tibbles and Tib do not share stages before the tail's loss, and they share stages afterward. The solution is the same as with splitting cases, except that the common stages are earlier rather than later.

The four-dimensionalist can say the same thing about the ancient case of the Ship of Theseus. Theseus hires a master shipbuilder to keep his ship in good shape. The shipbuilder repairs the ship as needed, saving all the parts he removes. When he has enough pieces, he begins putting them back together into ship form. Once all the original pieces are removed, he has a fully-constructed ship that he thinks is the original Ship of Theseus. But Theseus has had a ship all along that hasn't stopped existing just because parts have been removed. Which ship is the original? The temporal parts theorist insists that there are two ships across time, and each shares an initial stage with the other. The original-parts ship is a divided object that is disassembled for much of its existence, eventually coming back together. The continuous-ship ship is a ship the whole way through but changes its parts as it goes. Both ships exist, and both are ships (at least part of the time). Both can claim, in different senses, to be the Ship of Theseus. But there are really two worms here, and they both have the original ship-stage as their earliest stage. So it's another case of fission, like the above cases of the transporter accident and brain-hemisphere transplants.

The downside of four-dimensionalism is that it does seem to go against how we ordinarily see ourselves. I see myself as a wholly-present being who endures through time, and there aren't parts of me at other times that aren't here now. If four-dimensionalism is true, then that conception of myself is inaccurate. It's certainly possible for philosophy to clarify better ways of thinking about ourselves than we might have otherwise had, but how willing you are to accept such revisions to our thinking might depend on how well you think more intuitive approaches can handle the objections and whether you think other revisionist views can handle the problems better. Next up will be the conventionalist approach, which revises our conception of ourselves in a different way (although there are people who think both approaches are independently correct).

6 Comments

Woah. Awesome. I wasn't aware of this theory and I think I'm liking it.

In fact while it does seem kind of counter intuitive in some ways, I think it actually makes a ton of sense on a personal level. Afterall, don't we talk about "being a different person than I was back then". Who I am changes, sometimes in radical ways.

Also I maybe this solves the issues of moral responsibility? I've heard thought experiments like "A guy kills another man, and then gets hit in the head. He wakes up from a coma 6 years later and doesn't remember anything. Should he be punished?" The 4D understanding would say "no, because he is a different person now". Yeah?

3D and 4D views are in some ways independent of the criteria for personal identity. They're theories about what is involved when any object persists through time. In a sense, some of the personal identity issues are not answered by the 4D view, but this gives me a chance to say something about an issue I had left out of the discussion above.

What unites different stages into a bigger space-time worm? You could have a psychological view about that. You could have a bodily view about that. The most common 4D approach goes something like this. (This is how Ted Sider thinks of it.) The 4Der can accept that all the possible collections of 4D stages into worms are existing objects across time, but only some of them are interesting to us, and therefore only some of them are ones that we think about and refer to with language. Me up until 2000 and the Eiffel Tower from 2000-2010 is a genuine object on the level of ontology, but it's not one that's particularly natural, and naturalness makes objects more likely candidates for our terms and thoughts to latch onto.

Historically, there are a number of different things you can say at this point. What is it that unites the different stages into worms that we can speak of and think of as things? David Hume said there's nothing to unite the stages across time into larger 4D objects. That's an illusion. It's one we can't stop believing, but it's an irrational view that we're forced into by our biology. (He had a similar view on causation and scientific laws, which I talked about earlier in this series.) Immanuel Kant seems to have thought that the process of our thinking about unified objects across time is enough to make it rational, in our minds, to do so, with a kind of transcendental argument that it's required for our own unity of consciousness, which we can't do anything thinking without, and so we're forced to believe it on rational grounds just because we know we think. But on the basic ontological level, he seems to be just as skeptical of real unity of physical objects as Hume is. Jonathan Edwards even joins the ranks of skeptics on this question. He thinks God's decision to treat certain successions of stages as unified objects is the only thing that makes them unified. But the naturalness criterion for reference and thought that Ted Sider has most-clearly articulated is, I think, the most standard approach among 4Ders today, and it's far less skeptical than anything you'll see in the early modern 4Ders like Edwards, Hume, and Kant.

So you can have the Ed worm and the Fred worm as equally natural reference-candidates, and it's not just that both exist. We have reasons to refer to both. Then the statue-worm and the clay-worm are also both natural enough to think about and refer to. But human thought and reference doesn't make objects exist, and so objects that we don't normally have any reason to refer to do exist but just aren't ones we care about, talk about, or think about except when philosophers illustrate this theory with crazy examples. So there is such a 4D object as an in-car (an example from Eli Hirsch), which is a car-in-a-garage. It has a gappy existence. It goes out of existence whenever the car leaves the garage and comes back into existence when the car returns. The out-car comes into existence when the car leaves the garage and goes out of existence when it returns. You can speak of all this in terms of temporal parts or in terms of enduring 3D objects, but 3Ders hardly ever want to adopt a view like this (but a few 3Ders like this kind of thing). On a 4D view, the weird objects are more easily thought to exist, because all the stages exist, and the weird collections of stages are all sums of existing stages. They're just not unified in any natural way.

So on this view there is the guy who commits murder and then enters a coma, because there is such a thing as the sum of all person-stages in that body up to that point. Then there's also the guy who wakes up from a coma and doesn't remember doing it, because there's the sum of all person-stages after that point. But there's also the guy who is the sum of all stages in the body across its whole life. Which one is morally responsible depends on things besides just the metaphysics, but it does depend on what sort of metaphysical entity is a better candidate to be the bearer of moral responsibility and which features give continuity of moral responsibility over the course of a 4D object's perduring existence. 4Ders could disagree about those matters while agreeing on the basic ontological picture. A Lockean 4Der would base it on memory, but others might say that being a stage of the same body-worm is what counts. The next view we'll look at, contextualism, can combine with a temporal parts view as well, and on that view it may well be something to be determined by our linguistic (and mental?) conventions, but our society hasn't settled such matters, so there's no correct answer. That sort of view is popular among 4Ders.

So the short answer is that I don't think a 4D view does necessarily have anything particular to say about those moral questions, but you're right that it leaves open the kinds of things that you're suggesting. I don't see why a 3D view couldn't say the same things, though. Locke, for example, said exactly the sort of thing you say about the coma case, and I don't think he had a temporal parts view in mind.

Right that makes sense. I guess the 4D view still requires some kind of thread to link the worms, and in the example I gave I had the silent premise that memory was what was linking them, which could be just as well a 3D view. I do think that having the 4D element makes the memory argument more believable for me, because a 3D version is more open to skeptical questions about implanted or illusory memories. (and considering cog sci has been showing how memories aren't really a thing we have in our head, but rather a act which we perform each time we remember something, I think a 3D view tends to have ideas about the way memory works thats not entirely accurate. )

Why couldn't the same skeptical worries arise on a 4D view? If 4D is right, implanted and illusory memories could still occur. That's an issue of what happens to wholly-present objects or stages in the 4D worm, not an issue of whether it's wholly-present objects or 4D stages.

Illusory memories could occur but they wouldn't by themselves define self. So when I dream, I may be experiencing a set of non-existent memories in that dream, but just because I misremembered doing something I did, doesn't mean the memory itself constitutes a new identity since it is taken within the context of myself throughout the time worm.

But the same could be said with 3D. There's psychological continuity across time with a 3D being, and that's what makes me the same person I was yesterday and will be tomorrow.

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