What is the Basis of Object Identity?

Figure out your own best solution to the Ship of Theseus Problem

(Added 2/6/24)

Consider the difference between the statements "this is the same car" and "this car is the same". Suppose your car has been in the garage getting the engine overhauled and otherwise tuned up. When you go to the shop, the car you pick up is the same car you dropped off (otherwise, they won't let you take it away), but that car is not the same as it was better (it will perform better in various ways).  Or suppose your friend Jimmie Dale rolls up in a loud powerful car with all kinds of scoops, vents and spoilers. You say, "wow, you got a new car", but Jimmie says, no, this is the same car I had before, I've just had a lot of work done on it". When he says this, he is claiming that the car he has now, which is physically continuous with the car he had before, has the same object identity as that car he had before.

This topic is only about object identity. If you write about other forms of identity, you are off-topic.

Note: Further new material (in red) added on Oct 26, 2023.

Please make sure that you pay at least some attention to the idea that physical continuity (AKA "Spatio-Temporal Continuity") is actually the best solution to this problem. (I'm getting tired of reading papers that ignore physical continuity, or which assert that it isn't the basis of object-identity while actually relying on physical continuity to make their favorite theory seem to work.)

First, let me discuss the issue of object identity. Suppose your grandpa owns an antique pocket watch that he has willed to you. (Call this "watch 1".) Suppose he keeps it in a safety deposit box in a bank. Suppose also that, unknown to you and Grandpa, a man called Smith keeps an exactly similar watch (call this "watch A".) in another safety deposit box in the same bank. Suppose Grandpa periodically takes you in to the bank to look at his watch, so you get to know the appearance very well. Now finally suppose that your grandpa, alas, dies and two untoward things happen in the bank. First, a clerk accidentally drops Grandpa's watch and it receives a long scratch across the back (Call the watch as it exists at this time "watch 2".) and second, when you come in to pick up your bequest, the bank manager makes a mistake and hands you a different watch, (Call this "watch B".) a watch belonging to a man called Smith that just happens to exactly resemble Grandpa's watch in every respect, and so you take it to be Grandpa's watch, and for the rest of your life you carry this watch ("watch B".) around in the happy belief that this is the watch your grandpa owned, even though it actually is not the same watch.  Meanwhile, Smith collects the other watch, ("watch 2".) and, not noticing the scratch, takes away Grandpa's watch in the belief it is his own.

So there's two watches. There's the watch that Smith now holds, which is actually the same watch your grandpa used to own, even though it is different, owing to the scratch. And there's the watch you now hold, which is not the same watch your grandpa used to own, even though it is physically the same in every respect.

Does this story make sense to you? Or do you find yourself saying things like, "oh no, watch B is actually Grandpa's watch", or finding some other problem with the story.

When the Ship of Theseus problem asks, if some ship is the same ship as the ship Theseus used to sail, it is asking if it is that ship in the sense that the watch held by Smith, with the scratch, is the same watch that your grandpa used to own. It is not asking if it looks the same, feels the same, sounds the same or smells the same, it is asking if it is the same ship. (Remember, this is only about the basis of object identity, not social identity, or historical meaning.)

Physical Continuity

Before you start, you should get clear on the concept of "Physical continuity". This is the property of being the same coherent discrete object over time.

In the story of the two watches, watch 2 is physically continuous with watch 1, while watch B is not physically continuous with watch 1. In fact, physical continuity is the only thing that connects watch 2 with watch 1, so if you think watch 2 is watch 1, physical continuity is your only reason for thinking that they're the same watch.

Human bodies are physically continuous because although they grow and change over time, this is always because of small quantities of material, contained in food, drink, and air being added to the body, and other bits of material leaving the body as exhalations, discarded skin and hair, and other ways. Thus the body you have now is physically continuous with the body you had when you were born, and indeed, with the fertilized ovum you were just after you were conceived, even though the vast majority of atoms now in your body were not there at that time.

<Added 3/28/24> For instance, the 1894 Benz Velo in the Toyota automobile museum in Nagakute city has physical continuity with the 1894 Benz Velo that finished fifth in the world's first automobile race. (Most people would say that the one in the museum is the same car as the one that came fifth in the race.)

Another way to illustrate physical continuity is to consider two houses sitting side by side in a subdivision in 1950. Being recently made by very precise and conscientious builders, they are exactly similar to each other. They are so similar at this point that the only way to tell which one is which is by seeing (or remembering) where the other one is. While the houses are still indistinguishable from each other, they are purchased by Connor Tinuity and Samantha Form. Sam Form thinks that her new house is perfect. To her, everything about her new house, the facade, the floor plan, the colors, the doors, the windows, everything, is just exactly as she wants it. Con Tinuity looks at his new house and thinks, "it's a good start." Over the following years, a little bit at a time, Con makes many alterations to the house, moving windows, tearing out whole rooms, adding extra rooms, making a courtyard in the middle of the hourse, adding porches, adding a whole extra story, and a tower, and an observatory. This all takes a very long time, since Con only makes one change every year or so, but eventually, Con's house looks nothing like it's original form. In contrast, Sam works hard to keep her house exactly the same. Putting preservatives on its surfaces, fixing small problems immediately they arise, and so on. finally, in the year 2000, Con's house is a wild extravaganza that looks nothing like the original structure, and Sam's looks exactly the same as when she first bought it. Now, at this point, Con's house attracts tourist who come to gape at the strange house, and Sam haaaaaates the tourists, and haaaaates even looking at Con's house, and so she gladly agrees to sell her house to Con when he offers to buy it so he can put in a giant Tesla coil. Sam is especially happy to sell because she has found out that the builder who made her house also built several more exactly like it on the other side of town. Sam buys one of these, and moves all her stuff other there, setting up her new house exactly the way she had the old one. Meanwhile, Con has completely demolished Sam's old house, burning all the wooden bits and grinding everything else up for landfill. Now, the point of this story is to point out that Con's present house has physical continuity with his original house, even though it now looks nothing like his original house, and Sam's present house does not have physical continuity with her original house, even though it looks exactly.the same. This is what the term "physical continuity" means! If you have a house and you alter it in some way, say by moving a bit, adding a bit, or taking a bit away, the house after the alteration has physical continuity with the house before the alteration, even though the house is no longer the same.

If you're still not entirely sure what the issue is, this Video might help.

Practical solutions are preferred as opposed to arbitrary abstract, or speculative solutions that would never be used for ordinary objects in real life.

Alternate assignment: (Rewritten 11/3/22) Remember Grandpa's watch? Turns out that Smith, the owner of the other watch (the physically continuous with one you have) has found out about the switch, which is verified by security camera footage, and wants his original watch back. You don't want to swap, and don't want to believe that the watch you have isn't Grandpa's, so he takes you to court. You go into court with Smith, each believing that the watch they hold is your grandpa's watch, and that the watch the other holds is Smith's watch..

You hold a watch that is physically continuous with the unscratched watch Smith originally deposited at the bank for safekeeping.
Smith holds a watch that is physically continuous with the scratched watch Grandpa originally deposited at the bank for safekeeping.

Now, what rule should the judge use to decide which watch is yours, and which is Smith's?

Should he decide that the watch Smith has, the one that has physical continuity with the watch your grandfather owned is your grandfather's watch, or should he decide that the watch you have, the one that you believe to be Smith's watch, but Smith believes to be Grandpa's watch is Grandpa's watch?

<Added 4/4/24> To make things simple. I suggest that you think about whether physical continuity is the basis of object identity. If you decide it is, write a paper explaining both concepts, and saying why you think identity is based on continuity. If you decide it isn't, write a paper saying what you think is the basis of object identity, and why that (and not physical continuity) is the real basis of object identity.?

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