In Greek mythology, Theseus, mythical king and founder of the city Athens, rescued the children of Athens from King Minos after slaying the minotaur and then escaped onto a ship going to Delos. Each year, the Athenians would commemorate this by taking the ship on a pilgrimage to Delos to honour Apollo. A question was raised by ancient philosophers: After several hundreds of years of maintenance, if each individual piece of the Ship of Theseus was replaced, one after the other, was it still the same ship?
In contemporary philosophy, this thought experiment has applications to the philosophical study of identity over time, and has inspired a variety of proposed solutions and concepts in contemporary philosophy of mind concerned with the persistence of personal identity.
History
A Fresco from Pompeii depicting Theseus and Ariadne escaping from Crete. According to Plutarch, the Athenians preserved the ship that Theseus used to escape, by replacing the parts one by one as they decayed.
In its original formulation, the "Ship of Theseus" paradox concerns a debate over whether or not a ship that had all of its components replaced one by one would remain the same ship.[1] The account of the problem has been preserved by Plutarch in his Life of Theseus:[2]
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, Life of Theseus 23.1
Over a millennium later, the philosopher Thomas Hobbes extended the thought experiment by supposing that a ship custodian gathered up all of the decayed parts of the ship as they were disposed of and replaced by the Athenians, and used those decaying planks to build a second ship.[2] Hobbes posed the question of which of the two resulting ships, the custodian's or the Athenians', was the same ship as the "original" ship.[1]
For if that Ship of Theseus (concerning the Difference whereof, made by continual reparation, in taking out the old Planks, and putting in new, the sophisters of Athens were wont to dispute) were, after all the Planks were changed, the same Numerical Ship it was at the beginning; and if some Man had kept the Old Planks as they were taken out, and by putting them afterward together in the same order, had again made a Ship of them, this, without doubt, had also been the same Numerical Ship with that which was at the beginnings and so there would have been two Ships Numerically the same, which is absurd... But we must consider by what name anything is called when we inquire concerning the Identity of it... so that a Ship, which signifies Matter so figured, will be the same, as long as the Matter remains the same; but if no part of the Matter is the same, then it is Numerically another Ship; and if part of the Matter remains, and part is changed, then the Ship will be partly the same, and partly not the same.
Hobbes considers the two resulting ships as illustrating two definitions of "Identity" or sameness that are being compared to the original ship:
the ship that maintains the same "Form" as the original, that which persists through complete replacement of material and;
the ship made of the same "Matter", that which stops being 100 per cent the same ship when the first part is replaced.[3][4]
Proposed resolutions
The Ship of Theseus paradox can be thought of as an example of a puzzle of material constitution — that is, a problem with determining the relationship between an object and the material of which it is made.[1]
Constitution is not identity
According to the Stanford Encyclopedia of Philosophy, the most popular solution is to accept the conclusion that the material out of which the ship is made is not the same object as the ship, but that the two objects simply occupy the same space at the same time.[1]
Another common theory put forth by David Lewis, is to divide up all objects into three-dimensional time-slices which are temporally distinct, which avoids the issue that the two different ships exist in the same space at one time and a different space at another time by considering the objects to be distinct from each other at all points in time.[1]
Cognitive science
According to other scientists, the thought puzzle arises because of extreme externalism: the assumption that what is true in our minds is true in the world.[5] Noam Chomsky says that this is not an unassailable assumption, from the perspective of the natural sciences, because human intuition is often mistaken.[6] 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.[7]
Following on from this observation, a significant strand[who?] 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.[8]
Deflationism
According to the Stanford Encyclopedia of Philosophy, the deflationist view is that the facts of the thought experiment are undisputed; the only dispute is over the meaning of the term "ship" and is thus merely verbal.[1] American philosopher Hilary Putnam asserts that "the logical primitives themselves, and in particular the notions of object and existence, have a multitude of different uses rather than one absolute 'meaning'."[9] This thesis—that there are many meanings for the existential quantifier that are equally natural and equally adequate for describing all the facts—is often referred to as "the doctrine of quantifier variance."[10]
Continued identity theory
This solution (proposed by Kate, Ernest et al.) sees an object as staying the same as long as it continuously exists under the same identity without being fully transformed at one time. For instance, a house that has its front wall destroyed and replaced at year 1, the ceiling replaced at year 2, and so on, until every part of the house has been replaced will still be understood as the same house. However, if every wall, the floor, and the roof are destroyed and replaced at the same time, it will be known as a new house.[citation needed]
Alternative forms
In Europe, several independent tales and stories feature knives that have had their blades and handles replaced several times but are still used and represent the same knife. France has Jeannot's knife,[11][12] Spain uses Jeannot's knife as a proverb, though it is referred to simply as "the family knife", and Hungary has "Lajos Kossuth's pocket knife". Several variants or alternative statements of the underlying problem are known, including the grandfather's axe[13] and Trigger's broom,[14][15] where an old axe or broom has had both its head and its handle replaced, leaving no original components.
The ancient Buddhist text Da zhidu lun contains a similar philosophical puzzle: a story of a traveller 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, and the traveller was confused about who he was.[16]
Examples
Ships
USSConstitution is a modern Ship of Theseus, a historic warship which contains few or no original elements.
USSConstitution is acknowledged as a modern Ship of Theseus by the USS Constitution Museum. Since it was first laid down in 1794, the Constitution has undergone numerous repairs, modifications, and reconstructions.[18] Quoting from the website of the Antique and Classic Boat Society, "The USS Constitution has essentially none of its original wood but we believe no one would consider it a replica. It is Old Ironsides."[19]
The French critic and essayist Roland Barthes refers at least twice to a ship that is entirely rebuilt, in the preface to his Essais Critiques (1971) and later in his Roland Barthes par Roland Barthes (1975); in the latter, the persistence of the form of the ship is seen as a key structuralist principle. He calls this ship the Argo, on which Theseus was said to have sailed with Jason; he may have confused the Argo (referred to in passing in Plutarch's Theseus at 19.4) with the ship that sailed from Crete (Theseus, 23.1).
Aircraft
In 1911, the Vin Fiz Flyer was the first aircraft to fly coast-to-coast across the U.S., taking 49 days. It made 69 stops, of which at least 16 could be considered crashes. The aircraft had to be rebuilt multiple times; only a few pieces of the original plane actually made the entire trip.[20]
An upgraded aircraft can be a flying Ship of Theseus.[21] Regulators distinguish between changes to design (amendments to the type certificate) and the flight hours incurred by a particular aircraft (airworthiness certificate):[22]
The Boeing B-52H was manufactured during 1961–3, and subsequently received replacements to wings, skin, and avionics.[23] A planned (in 2024) upgrade will replace engines, radar, avionics, and make other changes. The United States Air Force regards these upgrades as worthy of a new designation, the B-52J.[24]
The Basler BT-67 is an aircraft remanufactured from a Douglas DC-3, where the latter may be as much as 80 years old (in 2023). Each remanufactured aircraft typically retains about 10 percent of the metal from the original aircraft, and receives new engines, new avionics, structural reinforcement, and other changes. Regulatory authorities regard the BT-67 as being a conversion of the DC-3 rather than a new type of aircraft, but also regard each aircraft as being brand-new with zero flight hours.[25][26][27]
Buildings
In Japan, the Ise Grand Shrine is rebuilt every twenty years with entirely "new wood". The continuity over the centuries is considered spiritual and comes from the source of the wood, which is harvested from an adjoining forest that is considered sacred.[28][29]
↑ Putnam, H., 1987, "Truth and Convention: On Davidson’s Refutation of Conceptual Relativism", Dialectica, 41: 69–77
↑ Hirsch, E., 1982, The Concept of Identity, Oxford: Oxford University Press. 2002b, "Quantifier Variance and Realism", Philosophical Issues, 12: 51–73.
↑ Laughton, John Knox. Memoirs of the Life and Correspondence of Henry Reeve, C.B., D.C.L. In Two Volumes., Volume 2. Hamburg, Germany: tredition GmbH. pp.Chapter XXIII. ISBN978-3-8424-9722-1.
↑ Knickerbocker, Brad (8 April 1983). "The B-52s: short on space, long on thunder". csmonitor.com. The Christian Science Monitor. Retrieved 19 January 2024. "It really isn't a 20-year-old airplane ... I would be surprised if there's an original rivet in any of those airplanes we have out on the ramp. It's been rewinged. It's been reskinned. It's been retailed. It's not an unsafe airplane. The airplane has been continually modified over the years. ... There are a number of systems in the airplane that are state-of-the-art." [including] the new offensive avionics system (OAS), which has begun to replace older analog computers and navigation components with modern, solid-state digital systems.
↑ Tirpak, John A. (23 June 2019). "B-52H May Become B-52J". www.airandspaceforces.com. Air & Space Forces Magazine. Retrieved 22 January 2024. Typically, the Air Force makes a letter-change designation to an aircraft—..."rolling the series"—when it receives enough new and different equipment that it constitutes virtually a new system
↑ Spaeth, Andreas (February 2023). "Making old new again: Reinventing the legendary Douglas DC 3". AEROREPORT. MTU Aero Engines. Retrieved 9 January 2024. The US Federal Aviation Administration (FAA) recognizes the reincarnations as brand-new aircraft with zero flight hours. ... At first it might seem an odd decision to laboriously convert old veteran fuselages instead of just building a new BT-67 from scratch. But there is a simple reason for this: cost. "Testing an entirely new aircraft and getting it certified is more expensive than our conversions that tend to use only the fully renovated original airframe,
↑ Jaroch, Walker (16 June 2022). "Basler Turbo Conversions Makes the Old New Again". aviationpros.com. Retrieved 22 January 2024. "We pull out all of the hydraulic lines, all of the interiors, all of the electrical wiring, everything comes out of the airplane, and it really is just the aluminum shell that's left, ... Every spar gets replaced regardless, just because of the age of it, and for the zero accumulated fatigue certification that we get from the FAA. ... it really is almost a complete replacement. ... We literally cut the airplane in half, and we stretch it 40 inches forward of the center wing. So our BT-67s are 40 inches longer than the C-47s were, and because those C-47s are 85 years old, now we gut it and replace literally everything inside the airplane, to the point that 90% of the aircraft is replaced in our conversion process,
Hobbes, Thomas (1656). "On Identity and Difference". Elements of philosophy: the first section, concerning body. London: R & W Leybourn. pp.100–101. Retrieved 24 September 2022.
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