Ship of Theseus
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? 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. History. Part of the thought puzzle (identity over time problem) was discussed by ancient philosophers such as Heraclitus ("Cratylus" 401d) and Plato ("Parmenides" 139), but there is no remaining evidence that they knew this paradox. The paradox was later discussed by Plutarch, 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: 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. 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. 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". 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. 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. 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. Chomsky says that this is not an unassailable assumption, from the perspective of the natural sciences, because human intuition is often mistaken. 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. 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. 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 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. 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. 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. In aviation, the remaining B-52H Boeing Stratofortress bombers are starting to become military aviation's "Ship of Theseus". 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.