Ship of Theseus

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The Ship of Theseus is a thought experiment about whether an object that has had all of its components replaced remains fundamentally the same object. Theseus was the mythical Greek founder-king of Athens, and the question was raised by ancient philosophers (e.g. Heraclitus and Plato): If the ship of Theseus were kept in a harbor and every part on the ship were replaced one at a time, would it then be a new ship?

Some follow-up questions are common: If it is not the same ship, then at what point did it stop being the ship of Theseus? If it is the same ship, then could all the removed pieces be reassembled to form a ship, and would that be the ship of Theseus?[1]

The concept is one of the oldest in Western philosophy, going back to the time of 500–400 BC. It is a common theme in the field of metaphysics.


Part of the thought puzzle (identity over time problem) was discussed by ancient philosophers such as Heraclitus (Cratylus 401d) and Plato (Parmenides 139),[2] but there is no remaining evidence that they knew this paradox. The paradox was later discussed by Plutarch,[3] and more recently by Thomas Hobbes and John Locke. Several variants are known, including the grandfather's axe and Trigger’s broom, which have each 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[3]

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.[4] Hobbes asked which ship, if either, would be the original ship of Theseus.

An ancient Buddhist text titled in Sanskrit Mahāprajñāpāramitopadeśa, which was later translated into Classical Chinese (Da zhidu lun 大智度論), contains a similar philosophical puzzle. It takes the form of a body-swapping story. The story tells of a traveler who encountered two demons in the night. As one demon ripped off all parts of the traveler's body one by one, the other demon replaced them with those of a corpse. The traveler was left confused about who he was after the body-swapping.[5]

Proposed resolutions[edit]

No identity over time[edit]

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 itself as it flows past. Arius Didymus quoted him as saying "upon those who step into the same rivers, different and again different waters flow".[6] 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".[7]


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.[8]

Cognitive science[edit]

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.[9] Chomsky says that this is not an unassailable assumption, from the perspective of the natural sciences, because human intuition is often mistaken.[10] Cognitive science would treat this thought puzzle as the subject of an investigation of the human mind. Studying this human confusion can reveal much about the brain's operation, but little about the nature of the human-independent external world.[11]

Following on from this observation, a significant strand in cognitive science would consider The Ship not as a thing, nor even a collection of objectively existing thing-parts, but rather as an organisational structure that has perceptual continuity.[12] When Theseus thinks of his ship, he has expectations about what parts can be found where, how they interact, and how they interact with the wider world. As long as there is a time/space continuity between this set of relationships, it is The Ship of Theseus. An organisational structure of course has to have components, but these also are defined in the same way. Such a recursive structure must "bottom out" somewhere and the enactivists[13] see this grounding to be based in our embodied relationship with our environment. In Cohen's (see below) example where a scavenger follows Theseus, collecting the discarded parts of the original Ship of Theseus, and then reassembles them, the reassembled ship is not The Ship of Theseus because, presumably a court of law would say, Theseus does not have the "owns" relationship with the reconstructed ship.


A literal example of a Ship of Theseus is DSV Alvin, a submersible that has retained its identity despite all of its components being replaced at least once.[14]

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.[15]

In aviation, the remaining B-52H Boeing Stratofortress bombers are starting to become military aviation's "Ship of Theseus".[16] As the electronics, wiring, sensors, metal skin, and rivets are upgraded or replaced, fewer and fewer components remain of the original aircraft that was built in the early 1960s. There will be few original B-52H components left when the Rolls-Royce F130 engines are installed in the late 2020s to replace the 1960s-era TF33s.[17][18]

Related concepts[edit]

A related concept is the concept of minimum viable vessel, e.g. what is the least amount of vessel (boat, ship, etc.) that qualifies to allow a person to enter a space that requires a vessel (e.g. a "no swimming" area) [19][20]

See also[edit]


  1. ^ Cohen, S. Marc (2004). "Identity, Persistence, and the Ship of Theseus". Retrieved 2019-03-15.
  2. ^ Plato (1925). Parmenides. Vol. 9. Translated by N. Fowler, Harold. London: Harvard University Press. p. 139.
  3. ^ a b Plutarch. "Theseus (23.1)". The Internet Classics Archive. Retrieved 2008-07-15.
  4. ^ De Corpore, ch 11.7
  5. ^ Huang, Jing; Ganeri, Jonardon (2021). "Is this me? A story about personal identity from the Mahāprajñāpāramitopadeśa / Dà zhìdù lùn". British Journal for the History of Philosophy. 29 (5): 739–762. doi:10.1080/09608788.2021.1881881. S2CID 233821050. An ungated version is available here or here.
  6. ^ Didymus, Fr 39.2, Dox. gr. 471.4
  7. ^ Plutarch. "On the 'E' at Delphi". Retrieved 2008-07-15.
  8. ^ 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.
  9. ^ 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.
  10. ^ Noam Chomsky (2010). Chomsky Notebook. Columbia University Press. pp. 9–. ISBN 978-0-231-14475-9.
  11. ^ James McGilvray (25 November 2013). Chomsky: Language, Mind and Politics. Polity. pp. 72–. ISBN 978-0-7456-4990-0.
  12. ^ Steve Grand (2003). Creation: Life and how to make it. Harvard.
  13. ^ Dave Ward, David Silverman and Mario Villalobos (18 April 2017). "Introduction: Varieties of Enactivism". Topoi. Springer. 36 (3): 365–375. doi:10.1007/s11245-017-9484-6. hdl:20.500.11820/cd543eb4-2ac5-4521-94eb-c39c43295840. S2CID 171748138.
  14. ^ Oberhaus, Daniel (12 December 2020). "The Oldest Crewed Deep Sea Submarine Just Got a Big Makeover". Wired.
  15. ^ Hosey, Lance (9 October 2018). "The Ship of Theseus: Identity and the Barcelona Pavilion(s)". Journal of Architectural Education. 72 (2): 230–247. doi:10.1080/10464883.2018.1496731. S2CID 115758753.
  16. ^ Axe, David (21 September 2021). "The U.S. Air Force Is Gradually Rebuilding Its B-52 Bombers From The Rivets". Forbes.
  17. ^ Axe, David (21 September 2021). "The U.S. Air Force Is Gradually Rebuilding Its B-52 Bombers From The Rivets". Forbes.
  18. ^ "Rolls-Royce North America Selected to Power the B-52 Commercial Engine Replacement Program". Rolls-Royce. 24 September 2021.
  19. ^ Crossing the Border of Humanity: Cyborgs in Ethics, Law, and Art Can Humans Being Machines Make Machines Be Human?, Cyborgs Conference, p47-64
  20. ^ Water-Human-Computer Interface Deconference, 2022, Publication date: March 30, 2022 DOI: 10.5281/zenodo.6403879, pages 5-22

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