Ship of Theseus

In the metaphysics of identity, the ship of Theseus is a thought experiment that raises the question of whether an object that has had all of its components replaced remains fundamentally the same object. The concept is one of the oldest in Western philosophy, having been discussed by the likes of Heraclitus and Plato by c. 500–400 BC.

This article is about the thought experiment. For the 2013 Indian film, see Ship of Theseus (film).

History

The thought puzzle was discussed by ancient philosophers such as Heraclitus (Cratylus 401d) and Plato (Parmenides 139),[1] later by Plutarch,[2] and more recently by Thomas Hobbes and John Locke. Several variants are known, including the grandfather's axe, which has had both head and handle replaced.

The particular "ship of Theseus" version of the thought puzzle was first introduced in Greek legend as reported by the historian, biographer, and essayist Plutarch:

The ship wherein Theseus and the youth of Athens returned from Crete had thirty oars, and 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 places, insomuch that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same.

— Plutarch, Theseus[2]

Plutarch thus questions whether the ship would remain the same if it were entirely replaced, piece by piece. Centuries later, the philosopher Thomas Hobbes introduced a further puzzle, wondering what would happen if the original planks were gathered up after they were replaced, and then used to build a second ship.[3] Hobbes asked which ship, if either, would be the original ship of Theseus.

Thought experiment

It is supposed that the famous ship sailed by the hero Theseus in a great battle was kept in a harbor as a museum piece, and as the years went by some of the wooden parts began to rot and were replaced by new ones; then, after a century or so, every part had been replaced. The question then is if the "restored" ship is still the same object as the original.

If it is, then suppose the removed pieces were stored in a warehouse, and after the century, technology was developed that cured their rot and enabled them to be reassembled into a ship. Is this "reconstructed" ship the original ship? If it is, then what about the restored ship in the harbor still being the original ship as well?[4]

Proposed resolutions

No identity over time

This solution was first introduced by the Greek philosopher Heraclitus who attempted to solve the thought puzzle by introducing the idea of a river where water replenishes it. Arius Didymus quoted him as saying "upon those who step into the same rivers, different and again different waters flow".[5] Plutarch disputed Heraclitus' claim about stepping twice into the same river, citing that it cannot be done because "it scatters and again comes together, and approaches and recedes".[6]

Four-dimensionalism
Main article: Perdurantism

Ted Sider and others have proposed that considering objects to extend across time as four-dimensional causal series of three-dimensional "time-slices" could solve the ship of Theseus problem because, in taking such an approach, all four-dimensional objects remain numerically identical to themselves while allowing individual time-slices to differ from each other. The aforementioned river, therefore, comprises different three-dimensional time-slices of itself while remaining numerically identical to itself across time; one can never step into the same river-time-slice twice, but one can step into the same (four-dimensional) river twice.[7]

Cognitive science

According to Noam Chomsky, as described in Of Minds and Language (2009), the thought puzzle arises because of extreme externalism: the assumption that what is true in our minds is true in the world.[8] This is not an unassailable assumption, from the perspective of the natural sciences, because human intuition is often mistaken.[9] Cognitive science would treat this thought puzzle as the subject of a biological investigation, as a mental phenomenon internal to the human brain. Studying this human confusion can reveal much about the brain's operation, but little about the nature of the human-independent external world.[10]

Gradual loss of identity

As the parts of the ship are replaced, the identity of the ship gradually changes, as the name "Theseus' Ship" is a truthful description only when the historical memory of Theseus' use of the ship—his physical contact with, and control of, its matter—is accurate. For example, the museum curator, before any restoration, may say with perfect truthfulness that the bed in the captain's cabin is the same bed in which Theseus once slept; but once the bed has been replaced, this is no longer true, and the claim would then be an imposture, because a different description would be more accurate, i.e.; "a replica of Theseus's bed". The new bed would be as foreign to Theseus as a completely new ship. This is true of every other piece of the original ship. As the parts are replaced, the new ship becomes exactly that: a new ship. Hobbes' proposed restored ship built from the original parts will be the original ship, as its parts are the actual pieces of matter that participated in Theseus's journeys.

Applications

The thought puzzle appears in several more applied fields of philosophy.

In philosophy of mind, the ship is replaced by a person whose identity over time is called into question.

In both philosophy of law and practical law, the thought puzzle appears when the ownership of an object or of the rights to its name are disagreed in court. For example, groups of people such as companies, sports teams, and musical bands may all change their parts and see their old members re-form into rivals, leading to legal actions between the old and new entities. Also, texts and computer programs may be edited gradually but so heavily that none of the original remains, posing the legal question of whether the owners of the original have any claim on the result.

In ontological engineering such as the design of practical databases and AI systems, the thought puzzle appears regularly when data objects change over time.

A literal example of a Ship of Theseus is DSV Alvin, a submarine that has retained its identity despite all of its components being replaced at least once.[11]

RNA, a central polymer of life, has been proposed to have evolved from an earlier genetic polymer in a manner that makes it a biological example of a Ship of Theseus.[12]

In architecture, Lance Hosey has proposed that the Barcelona Pavilion is "modern architecture’s ship of Theseus." Demolished and rebuilt in exacting detail on the original site, can it be considered the "same" building? He thoroughly documents the ambivalence of architects and historians about this question.[13]

Media
Main article: List of Ship of Theseus examples
  • In The Wonderful Wizard of Oz, the Tin Woodsman's origin story was that he was an ordinary flesh-and-blood man named Nick Chopper who gradually lost all his limbs, his torso, and finally his head, having each replaced by tin. The question of whether or not he remains the same person is brought up in a humorous fashion through the rest of the Oz series, culminating in the Tin Woodsman meeting a version of himself created from his flesh parts, with the two debating which of them is the "real" Nick Chopper.
  • In the Discworld novel The Fifth Elephant, King Rhys, the Low King of the Dwarfs, relates the story of how the original Scone of Stone has been replaced five times since the original dwarf bread disintegrated, making the replica the legitimate throne of the Low King. He offers his mining axe as proof of concept. Showing the lovingly crafted and maintained tool to Commander Vimes, he tells how it had once been his ancestor's axe; and when the axe's handle broke, it was replaced; and when the axe's head cracked, that also was replaced; yet, is it not still his ancestor's axe?
  • In the Futurama episode The Six Million Dollar Mon, Hermes is dismayed by his performance at work and his inability to protect his wife, and gradually replaces his body parts with robotic upgrades, brain last. As the parts are cast off, Zoidberg saves them to create a ventriloquists dummy. When the brain is finally added, Zoidberg has fully restored Hermes.
  • The name of the thought experiment was used as the name of the 2013 Indian drama film Ship of Theseus.
  • The ship of Theseus is referred to as the name of the fictional novel in Doug Dorst's novel S.
  • In the Warhammer 40,000 franchise, it is mentioned many human war machines such as tanks have had all their parts replaced due to constant battle damage over thousands of years of war. Nonetheless, the generations of pilots and crews all consider the vehicle to be the same object, even after every original part has been replaced.
  • In the video game Nier: Automata androids 2B and 9S help an android shopkeeper with a damaged leg to find parts. When asked why the shopkeeper does not just use parts to fix his broken leg instead of sitting in the village, the shopkeeper evokes the ideals behind the ship of Theseus. His damaged leg is his last original part. He questions: if he were to change it, would he still be the same person?
  • In the Only Fools and Horses episode "Heroes and Villains", Trigger wins an award for having owned the same broom for 20 years. He reveals that "this old broom, has had 17 new heads and 14 new handles in its time", but insists that it is still the same broom.
  • In the series Continuum where certain characters cause multiple versions of themselves to co-exist by time-travelling, the leader of the revolutionary movement Liber8 later becomes known to his followers as Theseus.
  • Ship of Theseus is the title of a 2020 Japanese TV drama produced by Tokyo Broadcasting System and starring Ryoma Takeuchi.
  • The 2012 fantasy film John Dies at the End depicts the grandfather's axe variant of this thought puzzle, when a man killed by an axe returns as a zombie to the protagonist's home after both the head and shaft have been replaced and proclaims "That is the axe that slayed me," to which the narrator responds: "Is he right?"

See also

References
  1. Plato (1925). Parmenides. 9. Translated by N. Fowler, Harold. London: Harvard University Press. p. 139.
  2. Plutarch. "Theseus (23.1)". The Internet Classics Archive. Retrieved 2008-07-15.
  3. De Corpore, ch 11.7
  4. Cohen, S. Marc (2004). "Identity, Persistence, and the Ship of Theseus". faculty.washington.edu. Retrieved 2019-03-15.
  5. Didymus, Fr 39.2, Dox. gr. 471.4
  6. Plutarch. "On the 'E' at Delphi". Retrieved 2008-07-15.
  7. David Lewis, "Survival and Identity" in Amelie O. Rorty [ed.] The Identities of Persons (1976; U. of California P.) Reprinted in his Philosophical Papers I.
  8. Massimo Piattelli-Palmarini; Juan Uriagereka; Pello Salaburu (29 January 2009). Of Minds and Language: A Dialogue with Noam Chomsky in the Basque Country. Oxford University Press. pp. 382–. ISBN 978-0-19-156260-0.
  9. Noam Chomsky (2010). Chomsky Notebook. Columbia University Press. pp. 9–. ISBN 978-0-231-14475-9.
  10. James McGilvray (25 November 2013). Chomsky: Language, Mind and Politics. Polity. pp. 72–. ISBN 978-0-7456-4990-0.
  11. Oberhaus, Daniel (12 December 2020). "The Oldest Crewed Deep Sea Submarine Just Got a Big Makeover". Wired.
  12. Hud, Nicholas V.; Cafferty, Brian J.; Krishnamurthy, Ramanarayanan; William, Loren Dean (18 April 2013). "The origin of RNA and "my grandfather's axe"". Chemistry & Biology. 20 (4): 466. doi:10.1016/j.chembiol.2013.03.012. PMID 23601635.
  13. Hosey, Lance (9 October 2018). "The Ship of Theseus: Identity and the Barcelona Pavilion(s)". Journal of Architectural Education. 72 (2): 230–247.


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