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 ca. 500-400 BC.
If it is supposed that the famous ship sailed by the hero Theseus in a great battle has been kept in a harbour 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, all of the parts had been replaced. The question then is if the "restored" ship is still the same object as the original.
If it is then supposed that each of the removed pieces were stored in a warehouse, and after the century, technology developed to cure their rotting and enabled them to be put back together to make a ship, then the question is if this "reconstructed" ship is still the original ship. And if so, then the question also regards the restored ship in the harbour still being the original ship as well.
No identity over time
This theory states that two ships, while identical in all other ways, are not identical if they exist at two different times. Each ship-at-time is a unique "event". So even without replacement of parts, the ships in the harbour are different at each time. This theory is extreme in its denial of the everyday concept of identity, which is relied on by most people in everyday use.
The concept of identity might then be replaced with some other metaphysical device to fill its role. For example, we might consider "Ship Of Theseusness" to be a property or class which is applied to all the events in the harbour as well as to the reconstructed ship-events.
This solution was first introduced by the Greek philosopher Heraclitus who attempted to solve the paradox 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". 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".
Continual identity over time via final cause
According to the philosophical system of Aristotle and his followers, four causes or reasons describe a thing; these causes can be analyzed to get to a solution to the paradox. The formal cause or 'form' (perhaps best parsed as the cause of an object's form or of its having that form) is the design of a thing, while the material cause is the matter of which the thing is made. Another of Aristotle's causes is the 'end' or final cause, which is the intended purpose of a thing. The ship of Theseus would have the same ends, those being, mythically, transporting Theseus, and politically, convincing the Athenians that Theseus was once a living person, though its material cause would change with time. The efficient cause is how and by whom a thing is made, for example, how artisans fabricate and assemble something; in the case of the ship of Theseus, the workers who built the ship in the first place could have used the same tools and techniques to replace the planks in the ship.
According to Aristotle, the "what-it-is" of a thing is its formal cause, so the ship of Theseus is the 'same' ship, because the formal cause, or design, does not change, even though the matter used to construct it may vary with time. In the same manner, for Heraclitus's paradox, a river has the same formal cause, although the material cause (the particular water in it) changes with time, and likewise for the person who steps in the river.
This argument's validity and soundness as applied to the paradox depend on the accuracy not only of Aristotle's expressed premise that an object's formal cause is not only the primary or even sole determiner of its defining characteristic(s) or essence ("what-it-is") but also of the unstated, stronger premise that an object's formal cause is the sole determiner of its identity or "which-it-is" (i.e., whether the previous and the later ships or rivers are the "same" ship or river). This latter premise is subject to attack by indirect proof[clarification needed] using arguments such as "Suppose two ships are built using the same design and exist at the same time until one sinks the other in battle. Clearly the two ships are not the same ship even before, let alone after, one sinks the other, and yet the two have the same formal cause; therefore, formal cause cannot by itself suffice to determine an object's identity" or " [...] therefore, two objects' or object-instances' having the same formal cause does not by itself suffice to make them the same object or prove that they are the same object."
One ship in two locations
In this theory, both the reconstructed and restored ships claim identity with the original, as they can both trace their histories back to it. As such they are both identical with the original. As identity is a transitive relation, the two ships are therefore also identical with each other, and are a single ship existing in two locations at the same time.
A basic principle of logical atomism is that facts in the world exist independently of one another. Only if we deny this principle then we can claim the following: the restored ship claims continuity of parts with the original over time and so, in the absence of other arguments, claims identity with the original. However when the reconstructed ship is completed and announced to the world, it presents a better claim on continuity, which changes the status of the restored ship making it lose its identity with the original. As a theory of observer-independent reality, this is hard to conceive; it involves both action at a distance and a violation of logical atomism. However it is more acceptable to Kantian style metaphysicists who view their subject as a theory of psychology rather than reality, as it described what biological humans are likely to believe in practice. (For example, if these were real ships on display to the public for a fee, it seems likely that the public would pay to see the reconstructed rather than restored ship.)
Definitions of "the same"
One common argument found in the philosophical literature is that in the case of Heraclitus' river one is tripped up by two different definitions of "the same", in other words the ambiguity of the term. In one sense, things can be "qualitatively identical", by sharing some properties. In another sense, they might be "numerically identical" by being "one". As an example, consider two different marbles that look identical. They would be qualitatively, but not numerically, identical; a marble can be numerically identical only to itself.
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, prior to any restoration, may say with perfect truthfulness that the bed in the captain's cabin is the same bed in which Theseus himself 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' 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 boat. As the parts are replaced, the new boat becomes exactly that: a new boat. Hobbes' proposed restored boat built from the original parts will be the original ship, as its parts are the actual pieces of matter that participated in Theseus' journeys.
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.
There is no ship
Ships do not exist. A "ship" is a label for a particular organization of matter and energy in space and time. The old "ship" is just a concept in the human mind. Similarly, the new "ship" (that has had all its parts replaced) is another concept in the human mind. If the two concepts were exactly the same, the human mind would not be capable of comparing them - there would be nothing to compare. Therefore, the old ship and the new ship must not be the same for the simple reason that humans are able to compare these two concepts against each other.
According to Noam Chomsky, as described in Of Minds and Language (2009), the paradox arises because of extreme externalism: the assumption that what is true in our minds is true in the world. This is not an unassailable assumption, from the perspective of the natural sciences, because human intuition is often mistaken. Cognitive science would treat this paradox 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.
The paradox had been discussed by other ancient philosophers such as Heraclitus and Plato prior to Plutarch's writings, 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, and the similar idea "Trigger's Broom".
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
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 used to build a second ship. Hobbes asked which ship, if either, would be the original Ship of Theseus.
The paradox 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 paradox 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 paradox 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.
- 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.
- The name of the paradox, "the ship of Theseus" is alluded to in the name of the 2013 Indian drama film Ship of Theseus.
- The ship of Theseus is referenced 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 doesn’t 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 busted 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 it has had 17 new heads and 14 new handles, but insists 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.
- Cohen, S. Marc (2004). "Identity, Persistence, and the Ship of Theseus". faculty.washington.edu. Retrieved 2019-03-15.
- Didymus, Fr 39.2, Dox. gr. 471.4
- Plutarch. "On the 'E' at Delphi". Retrieved 2008-07-15.
- 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.
- 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.
- Noam Chomsky (2010). Chomsky Notebook. Columbia University Press. pp. 9–. ISBN 978-0-231-14475-9.
- James McGilvray (25 November 2013). Chomsky: Language, Mind and Politics. Polity. pp. 72–. ISBN 978-0-7456-4990-0.
- Plato (1925). Parmenides. 9. Translated by N. Fowler, Harold. London: Harvard University Press. p. 139.
- Plutarch. "Theseus (23.1)". The Internet Classics Archive. Retrieved 2008-07-15.
- De Corpore, ch 11.7
- Quotations related to Ship of Theseus at Wikiquote