Cool Thought Experiments II: Ship Of Theseus
    By Garth Sundem | February 3rd 2009 05:00 AM | 36 comments | Print | E-mail | Track Comments
    About Garth

    Garth Sundem is a Science, Math and general Geek Culture writer, TED speaker, and author of books including Brain Trust: 93 Top Scientists Dish the...

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    Throughout history, scientists, philosophers, mathematicians and PhD students lacking funding for actual research have turned to the thought experiment in hopes of discovering something publishable, thereby retaining tenure and/or attracting the admiration of comely undergraduates.

    The best thought experiments throw light into dark corners of the universe and also provide other scientists, philosophers, mathematicians and destitute Phd students a way to kill time while waiting for the bus.

    Below is a classic thought experiment, pillaged from my book The Geeks' Guide to World Domination (Be Afraid, Beautiful People). I'll post a new thought experiment each day this week.

    Ship of Theseus

    The Greek historian Plutarch described the following dilemma: “The ship wherein Theseus and the youth of Athens returned was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their place.”

    By the time of Demetrius Phalereus, which was at least 200 years after Theseus’ return from Crete, so many planks and timbers had been replaced that none of his ship’s original wood remained.

    The question is—was it still Theseus’ ship? More generally, what creates identity? If all the molecules of a thing (or person) are identical to the molecules of another thing (or person) are the two the same? If they are different, what makes them so? If a person were teleported by a machine that disintegrated their molecules and then reassembled them in an exact copy, would it be the same person?

    What d'you think?


    Hi, liked your blog so I decided to come back eheh.

    The ship is and isn't the same depending on the perspective we have:
    -if we consider only matter free from any human significance it is not the same because matter has changed (which microscopically speaking raises the problem that we might not be the same we were X time ago);
    -if we otherwise consider any human significance the problem is hard to solve because we need to accept human significance(emotional, historic,...) as an attribute of an object, and also that an object remains the same until all its attributes have changed;

    In the case of the teleportation the problem is the same but we have a subjective component in the human case. So if I were teleported in a way that my 'original' molecules were destroyed would I die or would I be alive in the copy?
    What if the molecules were copied and the original not destroyed who of the two would be really myself?
    In my view if we can distinguish two things they are not objectively identical but subjectively things are harder than that and I think that I am not (yet) capable of answering.

    Gerhard Adam
    From a philosophical perspective, the mere fact that the question can be asked already indicates that it is NOT the same (in the ship or teleportation case), since such a question is meaningless when only an original is considered.

    There is certainly a "sameness" about it, just as one would have with a copy, but it isn't the same.  For example, if we considered using the new ship, we might find that it is more seaworthy, or leakier, or whatever.  In noting such a characteristic we would be forced to consider that whatever its behavior, it could be due to the fact that it was no longer the same ship used originally.

    One of the quotes indicates that newer and stronger wood was used, but why stop there?  Why not use steel when it becomes possible?  or fiberglass?  The point is that the similarity between the "new" and the "original" is being superficially maintained but they are not and cannot be the same vessel.

    In particular, the "samenss" being attributed to the vessel is an attempt to freeze it in time to commemorate a particular event.  However, it is useful to consider what it would mean if Theseus had kept the ship and maintained it himself.  Would the improvements, repairs, and evolution of the ship constitute the same vessel some years later?  In truth, it would be more Theseus' ship than the maintained copy, but in both cases, it would not be the ship originally employed.

    The question is also raised about what creates identity, but this is also a misleading question because it isn't identity, but rather "identical" which is being questioned.  There's no question we can all recognize a human hand, but it would hardly be reasonable to say that they are not fundamentally identical.   We can also see differences, but they are not significant enough to cause identity problems.  In the same vein, one has to consider that the changes in the wood for the ship represent the same kind of changes which allows us to identify a hand and yet recognize that they are not all the same.

    Consider whether Theseus's ship would still be significant if it was a myth that it had ever been used?  Which would have a more profound effect?  The changing materials, or the story that accompanied it?
    Mundus vult decipi
    I'm going to have to disagree with your conclusion, but only to an extent, and first I want to point something out.

    The answer to the question is very dependent on the definitions you use. By the definition you've taken, even once the first plank is replaced, it's not the "same" - to quote you, "we might find that it is more seaworthy, or leakier, or whatever." You are, of course, entirely right - given a strict definition of "the same", the boat isn't the same even from one moment to the next, much less after you replace that first plank.This question isn't valuable because there's a right answer, it's valuable precisely because there isn't one.

    Seeing the problem from another perspective, each new plank becomes a part of the boat, and the removed plank is no longer a part of the boat. When you replace the final original plank, you're doing precisely that and no more, much like when the last vestiges of the cells that made up the infant I once was have been replaced, you can say that there's not a cell of the original me there, but I'm still the "same person", from a strict identity point of view, even though I'm now vastly different.

    No amount of philosophising, I suspect, could make me comfortable with the hypothetical idea of macro-scale teleportation; the gradual repair of the ship (or the body) is a case of discarding the dead wood piece by piece, preserving the identity in the 99+% of the entity which remains intact throughout the repair, while the new part becomes a part of the ship. In the teleportation case (depending on the hypothetical method, I suppose) we're deconstructing the entirety of the original to re-create it elsewhere. This might make sense with a ship (or a bridge) if we were physically transporting the planks (or the bricks), but consider if we were simply pulling it apart for the plans, and re-building with all new materials! The idea leaves me feeling profoundly uncomfortable were we to consider it with a living thing - again, not because of the answer to the question, but because of the discussion it provokes.
    Perhaps this thought experiment most clearly delineates the difference between scientists and engineers. For example, a scientist would have dating difficulties (dating befuddlement) if he or she finds that only half the distance is gained per time interval towards a suitable romantic object. The engineer's romantic goal would be fulfilled when achieving a distance 'suitable for all practical purposes'. Perhaps like Eskimos and snow we don't have a rich enough language, so 'identical' is just too general for any practical purposes.

    Becky Jungbauer
    I'm with Gerhard on this - it isn't the same ship. The very use of the word "new" and "stronger" implies different. In Book X of Plato's Republic, Socrates and Glaucon address imitation in a similar vein:

    Now do you suppose that if a person were able to make the original as well as the image, he would seriously devote himself to the image-making branch? Would he allow imitation to be the ruling principle of his life, as if he had nothing higher in him?

    I should say not.

    The real artist, who knew what he was imitating, would be interested in realities and not in imitations; and would desire to leave as memorials of himself works many and fair; and, instead of being the author of encomiums, he would prefer to be the theme of them.
    I like that idea - that while Theseus' ship had been updated and was not the same as the original, the ship was still "his" in theme, myth, legend, idea.
    Steve Davis
    Nicely put Becky. The updated ship still had something of Theseus about it, while a ship reconstructed from a plan fails to capture most of that essence.
    Becky Jungbauer
    Essence - that captures it well, Steve.
    Garth Sundem
    Damn, there are some smart readers on this site. Nice job all. And I'm sorry I wasn't at my keyboard today to see the fireworks. And believe you me, as my excuse was spending a day at Disney with two kids under three, I'm truly, very very sorry.

    The consensus I seem to see here is that Theseus' "new" ship (with replaced timbers) is different from the original. It seems that one argument being made is that an item's history becomes part of its identity (yes, I just used the word "identity" which I see has a distinctly different meaning than "identical"...). This, even if the "new" or "identical" object thinks it owns this history—in other words, if a teleported person arrived with all his/her memories and scars intact.

    Hmmmm. I'm not sure what I think about this. And I guess that's why we've been talking about it since the time of Theseus.

    Join me tomorrow? Same bat time, same bat channel.


    Garth Sundem, TED speaker, Wipeout loser and author of Brain Trust

    Garth Sundem
    Here's another question: in the situation Nuno Cravino described in which a person is replicated exactly, would the copy necessarily make the same decisions as the original? Would the two, given the exact same circumstances, act the same?

    Garth Sundem, TED speaker, Wipeout loser and author of Brain Trust

    Gerhard Adam
    I would argue that they wouldn't.  Even if everything down to the quantum level were replicated, the fact that there were two of them (or at least knowledge that one had been replicated) would be enough of a difference to alter the potential future behavior or actions.  In other words, the knowledge of the replication itself is a major difference that cannot be undone.
    Mundus vult decipi
    Well if it was possible that they are fully replicated, and if it was possible to put them under the same circumstances, I would almost bet they would act in the exact same manner, at least looking at the problem in a classical deterministic way.
    Otherwise if we accept the indeterminism from quantum physics as ontological and not merely subjective well the two of them can act whatever way they want, and it would be a real miracle to have them replicated exactly and under the same circumstances.

    Garth Sundem
    Cool point, Gerhard. What about the abstract situation in which the copies don't interact? If there are two exact copies in exact situations, independent of each other? And you're right, Nuno, I know this is slipping into the old debate of free will vs determinism, which doesn't actually have an answer, but it's a cool lens through which to explore it, eh?

    Garth Sundem, TED speaker, Wipeout loser and author of Brain Trust

    The concept of infinity was invented to allow for the possibility that two clones under identical circumstances will act the same way.  And the guy who invented infinity didn't even know anything at all about a quantum universe.

    Now, matter is not infinitely divisible so there's no reason energy should be.  That means at some level you should be able to duplicate everything precisely.    The problem is, indeterminacy says we could never know.

    Schrodingers cat lolcat
    Gerhard Adam

    I don't think it matters whether they interact or not.  It seems obvious that they would have very similar behaviors since the sum of all the component parts would be very similar, but identical is problematic since even a slight displacement in space/time would also slightly modify whatever is being experienced, which would slightly modify any reaction to it.  Therefore complete replication would also require occupying the same position in space and time to be identical.  Anything short of this would represent a different condition and consequently give rise to different results.

    The fundamental problem with teleportation, as stated, is that we are presuming that a complete copy of an individual is taken apart and reassembled at some other point.  However, since the information must be stored in some intermediate location, then clearly there is nothing preventing us from creating multiple copies.  As stated previously, since none of them could occupy the same space and time simultaneously, they would all be subject to different "initial conditions" and therefore their behavior would be divergent.  In short, they would all have the same "tools" but they would all be different.

    I have to conclude that even the single teleported individual is no longer the same.  Since the information about the individual is stored (as indicated in the previous paragraph), then clearly it can also serve as a back-up.  Therefore it would be possible to construct this individual at any point in time.  It would also be possible upon the individual's death to simply "restore" them to the last back-up.  Once we accept the idea that an individual can be assembled in this fashion, then all questions surrounding individual existence become irrelevant.

    Mundus vult decipi
    Becky Jungbauer
    A thought - what if there was only one 'person', be it the original or the copy? Nuno writes, "So if I were teleported in a way that my 'original' molecules were destroyed would I die or would I be alive in the copy? What if the molecules were copied and the original not destroyed who of the two would be really myself?"

    In the second instance I'd agree with Gerhard, if there was knowledge of the copy, and Hank in either case. As for the first - if there is only one, wouldn't that one have all of the memories, the molecules, the essence? It wouldn't make a difference, then, unlike the ship, which had new parts replacing the old. This reminds me of The Prestige, in which Robert Angier (Hugh Jackman) harnesses the genius of Nikola Tesla (David Bowie) to best Alfred Borden (Christian Bale). Angier delves into teleportation, and Borden has a way to mimic himself as well (but if you haven't seen the movie I won't ruin the surprise). Anyway, the movie deals with this crisis of 'identity' and how originals and/or copies deal with the knowledge of the other.
    I think this paradox falls into thinking things are too concrete-- something being either 'is' or 'is not'. I've also heard this as 'my grandfather's axe', as in "I still have my grandfather's axe. I've replaced the blade twice and the handle once, but it's his axe".

    The corrollary is, if you had a rusty, dull axe with a cracked handle, would your grandfather consider it 'his axe'?

    So if you define Theseus's ship as "a shipworthy item Theseus sailed", that no longer exists-- whether you'd replaced stuff or not, it's lost to time. So you can either choose from "Theseus's ship+", the updated and restored item, or "Theseus's ship-", the remnants of the original. We have thus have three possibilities:

    a) The original, no longer exists.
    b) The best restoration, an accurate clone of the original, or similar conceptual duplicate-- the functional descendent of the original.
    c) The remains of the original, an artifact but no longer a functioning 'ship'

    But then also, what is Theseus's ship? Is it the original ship he bought, or his ship after he got new sails, or after he modded it with those bitchin' new oars? I'll argue there is no single "Theseus's ship". Part of the fallacy of ownership is a presumption that nothing changes and ownership somehow fixed the concept into unmutable reality.

    In terms of people, are you the same "you" as you were 10 years ago? I would argue that, much as Theseus' ship, there is no single entity you can point to from the past that still exists now. Every day, you're either a modification, restoration, or remains (depending on your point of view) of the earlier "you".

    And if you want to get into too much relativism, you can think about movies-- which version of the movie is 'authentic'-- the original release, the later rerelease, the DVD edition, the director's cut?

    So the solution to the paradox is that you have to adopt a definition of what you mean by "Theseus's ship"-- is it the pieces that made the original, the functional continuation of, or a lifelike simulation? Because the original "Theseus ship" was itself a constantly changing entity, and there never was a single Platonic ideal "Theseus's ship"-- even to Theseus.


    Garth Sundem
    Nicely put, Alex. I'm curious what you might have to say about thought experiment IV:The Chinese Room.

    Garth Sundem, TED speaker, Wipeout loser and author of Brain Trust

    Same ship I think. Like a living thing, shedding off or acquiring constituyents but still the same differenciated, unique entity. Only a parallel or replacement ship would void the idea.

    It is Theseus re-modeled ship. If I replace a doorknob of my home, a door, carpet, etc. until everything in the house is re-modeled, it is still "MY" house. However, if my son, after I die, replaces everything in the house, it is not my house, instead it is my son's house.....where I used to live. Therefore, it is a boat that Theseus used to sail on that has been completely re-modeled, but it is no longer Theseus boat.

    "Now, it is a law in Lloyd's that the Jane repaired all out of the old until she is entirely new is still the Jane." Joshua Slocum, in "Sailing Alone Around the World." This is really a question for the cognitive scientist: "Under what circumstances, if ever, will a named entity cease to be named as that entity?" My own view is that there is a cognitive resistance to the acceptance of any change that requires a long-established mental category to be re-labelled. A common example: in all cultures there is a tendency for friends and relatives to continue to refer to 'the deceased' by personal name/s. Especially amongst sailors, there is a strong tendency to personify a ship and there is a great sadness over its loss. There are, of course, exceptions. The wreck of the liberty ship Richard Montgomery in the Thames estuary is known locally simply as 'the wreck'. For Athenians, Thesius' ship would have had great emotional significance. It follows that there would have been a reluctance on every citizen's part to admit that the original had somehow become a replica. Finally, I will throw out for discussion HMS Victory and the Cutty Sark - are they still the same ships that once made the British proud?
    its mass of course!

    I have no stomach! It's true. The stomach cells that I was born with have been replaced (by this time in my life, multitudinous times) with newer cells. All of the old cells have been expelled through my digestive system with all of the other solid and liquid toxins (poo and pee) that my body doesn't want. Not a single cell from my nascent stomach is still there today. (In fact, this is precisely why we're able to digest food. Our stomachs are continually regenerating.)

    Does my stomach stop being my stomach when it is no longer made of the stuff that it was originally made of?

    Not really, I just had a decaf coffee with artificial sweetener and a non-dairy creamer, making it more science experiment than Colombian delight.  It was still my cup of coffee.
    Gerhard Adam
    I think you're using a questionable example.  Would any stomach do? or is there something specifically unique about yours?  Obviously the answer is that it isn't the specific cells that define your stomach, but rather your DNA.  So, there is something unique about your stomach that makes it specifically yours.

    Similarly there is something unique about an object that is associated with a specific event in history.  If you wanted to see the Declaration of Independence, you don't want to simply see a copy of it.  That has no value.  It is only desirable if you can view the original and all the psychological and cultural effects that generates.

    If I go visit a historic site, I don't want to see a 21st century replica of what was there.  Would you want to go to Egypt and see a replica of the Great Pyramid?  If I can't see the original, then what's the point?  I might as well go to DisneyWorld and watch the Tiki Birds sing songs. 
    Mundus vult decipi
    I might as well go to DisneyWorld and watch the Tiki Birds sing songs.
    Gerhard: didn't you know?  The original Tiki Birds have been replaced!
    The Enchanted Tiki Room is an updated version of the original Tiki room with two new birds who have taken over the shop.
    Gerhard Adam
    Is nothing sacred?
    Mundus vult decipi
    They say you replace every cell in your body every 7 years or something. So if it isn't the same ship, then neither are you the same person you were 7 years ago. We can't give jail terms longer than 7 years, because you'd be punishing a different person after the 7 years are up.

    Gerhard Adam
    Not true.  There are many cells that do not reproduce.  More importantly, you're presuming that a "person" is the same thing as the "organism".  Your jail term statement is simply silly, by any definition.
    Mundus vult decipi
    Well, it's true for most of the cells in your body. And even the cells it is not true for are in large part not made of the same atoms they were 10 years ago.

    My feeling about the ship thought experiment is that it is the same ship, and of course the jail hyperbole is an illustration of how in human affairs, the replacement of components does not affect identity.

    Gerhard Adam
    Part of the reason that your identity isn't affected is because the neurons don't reproduce.

    In any case, there's a problem with your comparing this to the ship, because you can always be confident that the cells will reproduce in the same manner they always have within your body.  The same can't be said for the ship, where different tools and different technology will be applied for carpentry or machining.  Therefore, if the techniques used to manufacture the replacement parts isn't the same, then at which point does it simply become a model of the original?

    Take it from another perspective.  Suppose we just built the ship using modern technology, but made it exactly as the original?  Would it be considered ancient?  or a model replica?

    If the latter, then why would it not be a model replica if all the constituent parts had been replaced during restoration?

    I think part of the point is that we know what these ancient ships or structures look like, so it isn't a problem of building them or replicating them.  It is in the apparent history of the parts that it becomes significant.  If none of the parts are actually historical, then what's the point?  We like to look at things an imagine who made them, or what was experienced.  If it's all modern, then there's no purpose to it except to illustrate the replica. 

    As a final thought ... what you want to see the pyramids as they were originally built, or would you want to see them restored to their original state?  Which is more significant to you?
    Mundus vult decipi
    Cells reproduce in my body such that it gives my identity continuity. Note that I'm not even superficially identical to what I was 10 years ago. Similar, but now aged. In the same way, one assumes that the ship in the thought experiment was repaired on an as needs basis that more or less gave the ship some continuity. It didn't start out as a row boat and end up as the titanic. It was probably still recognizable, even if not identical.

    Of course, you could take this to the point of absurdity, and ask what if all the parts were replaced overnight. But I don't think the pathological case disproves the normal case.

    You ask about the pyramids, do I want to see them as original or as they are now. My general philosophy is that buildings should be kept in a state of repair. What if, hypothetically the pyramids had been continually kept repaired over 3000 years such that they still looked pretty similar to original? Would you advocate now ceasing repairs?

    With the Greek Parthenon, most of the damage is actually relatively recent. I wouldn't see a problem repairing it, trying to keep as much original as possible. If they keep repairing it, it might still be around in another 3000 years. If none of the parts are historical, what's the point? So must we let everything of worth crumble into disrepair lest it be accused of not being historical?

    Gerhard Adam
    It depends on what the purpose of preservation is.  If it is merely to illustrate what things were like, then by all means, repair away.  If the intent is to actually maintain the original, then if it crumbles, it crumbles.

    This is precisely why paintings are specially handled to maintain their integrity.  It wouldn't be worth much if we simply had a modern artist keep touching it up and "repairing it".  Similarly, it wouldn't be worth much if we simply digitized the image and put it on a CD.

    There is a point to what elements you want to maintain the integrity of and then you have to go with that if it is to maintain that meaning.
    What if, hypothetically the pyramids had been continually kept repaired over 3000 years such that they still looked pretty similar to original? Would you advocate now ceasing repairs?
    Obviously not, because part of their history would include their continued maintenance.  Similarly we can consider whether something that is historical should necessarily be preserved.  Once again, it all depends on context.  If I find primitive stone tools, then I certainly would want them to be in their original state and preserved accordingly.  I don't want someone making a modern replica.  On the other hand, if I find a home that is a few hundred years old, in ruins, there's not necessarily any reason for me to not tear it down.

    You can see that there are a variety of ways that this can play out.

    In short, according to the question, it is NOT Theseus’ ship, although it is a completely restored replica.  While someone may argue that this is nit-picking, then why not simply say that it is Theseus's ship but that it no longer has any of the original building materials in it?  Do you think people would find it as interesting? 
    Mundus vult decipi
    You say had the pyramids been maintained, then we should continue doing so. But, they may well have been maintained for 500 years, and then fell into disrepair. How long do they have to be in disrepair before we give up? What if we repaired them now, and then they stood in a state of repair for another 100,000 years? Would the 2000 years in disrepair matter in that context?

    And are you saying that we can't repair buildings with the goal of maintaining the original, we can only repair with the goal of making a fake of the original? We must let everything from the Eiffel tower to the Sydney Opera house crumble, if we want to have the "real thing", and not just a neat fake?

    And modern artists do keep touching up and repairing old masterpieces. Obviously they do it as little as possible, but when they are damaged, they do get touched up.

    Is Theseus' ship with no original building materials as interesting as the unadulterated Theseus' ship? Well, yes and no. There would be a certain integrity if every interesting item were put into a crypt and held there for posterity. But then part of the interest of items is their history of usage, and things that are never repaired but simply left to deteriorate don't end up acquiring much of a history. So Theseus' ship in continual usage is pretty interesting in itself, because it would have had an even longer and more illustrious history than if it was stuck into stasis.

    Gerhard Adam
    You're missing the point.  When we look at something that is claimed to be "original", we don't expect that it's been tinkered with.  Regardless of the reasons, there may be an opportunity to have restoration with a minimal amount of modification, on the other hand, sometimes this simply isn't possible.  I get that.

    My point is simply that if we restore something, then it should be clearly understood what's been restored or maintained, so that people don't mistakenly assume that what they are looking at is "original" [since it isn't].

    I suppose in many ways, the issue isn't just the item, but the workmanship that went into it.  Therefore if we only intend to display the item, then any amount of restoration and preservation is acceptable.  If people are looking at it regarding ancient workmanship or skills, then we can't. 

    So back to Theseus's ship.  If the purpose is simply to show the ship, then by all means, restore away.  On the other hand, it should be clear that one can draw no conclusions about the workmanship or skills of people that built it, if it has been modified/restored/"corrected" by modern workers [without the clear understanding of the level of restoration involved].

    In countless displays, people go to see [and pay to see] original items and not copies.  Whether it be to look at the Declaration of Independence or one of Jimmy Page's outfits at the Rock'n'Roll Hall of Fame.  Interest exists because they are originals and not merely copies. 
    Mundus vult decipi
    Well, nobody mentioned "original" up till now. That's a word with its own particular set of connotations. And "drawing conclusions" about the state of the ship originally Is also a different issue. That's like looking at an aged Muhammad Ali and saying that can't be the same Muhammad Ali who was world champion because clearly this person would not be capable of such a feat. The attributes of things can change over time and yet still be the same item. It's the continuity of usage that gives it identity along with some degree of continuity of makeup.

    Gerhard Adam
    Well, you're attaching a different meaning to the word "original" then.

    1. Preceding all others in time; first.
    2.a. Not derived from something else; fresh and unusual: an original play, not an adaptation.
    b. Showing a marked departure from previous practice; new: a truly original approach. See Synonyms at new.

    1. A first form from which other forms are made or developed: Later models of the car retained many features of the original.
    2.a. An authentic work of art: bought an original, not a print.
    b. Work that has been composed firsthand: kept the original but sent a photocopy to his publisher.

    If that's not the basis for this discussion, then what is?  Not much point in these considerations if its only about copies.
    Mundus vult decipi