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 ca. 500–400 BC.

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?

Proposed resolutions

No identity over time

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".

Four-dimensionalism

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 river twice.

Cognitive science

According to Noam Chomsky, as described in Of Minds and Language, 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.

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.

History

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.
This particular version of the paradox was first introduced in Greek legend as reported by the historian, biographer, and essayist Plutarch:
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.

Applications

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.

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