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 Heraclitus and Plato by c. 500–400 BC.

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 puzzle itself is an example of Ship of Theseus, as the original story has had various elements replaced over time.

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.

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 his 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.^[4]

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 whether 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?^[5]

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 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]

Four-dimensionalism[]

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[]

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.

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.^{[citation needed]}

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.^{[citation needed]} 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.

Yes, Blackfoot, Blood, Sweat and Tears, The Flying Burrito Brothers, Thin Lizzy and Ratt are examples of rock bands that have retained their names despite member reshufflings to the degree that none of the founding members of the respective bands remain.

In ontological engineering such as the design of practical databases and AI systems, the thought puzzle appears regularly when data objects change over time.^{[citation needed]}

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] A similar example is the USS Constellation which may have been rebuilt using pieces from the original ship.

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.^[15]

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.^[16]

Media[]

There are many explicit or implicit references to the Ship of Theseus in books, television, film, music, software, and games.

See also[]

References[]

External links[]

• Quotations related to Ship of Theseus at Wikiquote