Ship of Theseus
The ship of Theseus, also known as Theseus's paradox, 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 paradox is most notably recorded by Plutarch in Life of Theseus from the late first century. Plutarch asked whether a ship that had been restored by replacing every single wooden part remained the same ship.
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.
Variations of the paradox
The classic paradox
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, in so much 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 contemporary writer, Martin Cohen, creatively imagined John Locke regarding a favorite sock that develops a hole. He pondered whether the sock would still be the same after a patch was applied to the hole, and if it would be the same sock after a second patch was applied, and a third, etc., until all of the material of the original sock has been replaced with patches.
George Washington's axe (sometimes "my grandfather's axe") is the subject of an apocryphal story of unknown origin in which the famous artifact is "still George Washington's axe" despite having had both its head and handle replaced. This has also been recited as "Abe Lincoln's axe"; Lincoln was well known for his ability with an axe, and axes associated with his life are held in various museums.
In Europe, there are several tales and stories of knives that have had their blades and handles replaced several times independently, but are still used and represent the same knife. France has Jeannot's knife,  Spain uses Jeannot's knife as a proverb, though referred to simply as "the family knife", and Hungary has "Lajos Kossuth's pocket knife".
In Japan, Shinto shrines are rebuilt every twenty years with entirely "new wood". The continuity over the centuries is spiritual and comes from the source of the wood in the case of the Ise Jingu's Naiku shrine, which is harvested from an adjoining forest that is considered sacred. In 2013, the shrine was rebuilt for the 62nd time.
One version is often discussed in introductory Jurisprudence and Evidence classes in law school, discussing whether a weapon used in a murder, for example, would still be considered the "murder weapon" if both its handle and head/blade were to be replaced at separate, subsequent times.
In popular culture
The paradox appears in various forms in fictional contexts, particularly in fantasy or science-fiction, for example where a character has body parts swapped for artificial replacements until the person has been entirely replaced. There are many other variations with reference to the same concept in popular culture for example axes and brooms.
The Greek philosopher Heraclitus 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".
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 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."
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 vagueness 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.
Note that some languages differentiate between these two forms of identity. In German, for example, "das Gleiche" ("equal", it can be a copy of the object) and "dasselbe" ("self-same", original, not a copy) are the pertinent terms, respectively.    At least in formal speech, "I have bought das Gleiche smartphone, except mine is white" (same make and model), but "This is definitely dasselbe smartphone that was stolen from Irene, it has the same serial number and IMEI, it has her contacts, and its mobile WIFI is even named "Irene's smartphone"; so it is definitively dasselbe smartphone!"
Colloquially, "das Gleiche" and "dasselbe" are used somewhat interchangeably.
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.
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|Wikiquote has quotations related to: Ship of Theseus|
- 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
- Cohen, M. (2010). Philosophy for Dummies. Chichester: John Wiley & Sons.
- Browne, Ray Broadus (1982). Objects of Special Devotion: Fetishism in Popular Culture. Popular Press. p. 134. ISBN 0-87972-191-X.
- "Atomic Tune-Up: How the Body Rejuvenates Itself". National Public Radio. 2007-07-14. Retrieved 2009-11-11.
- Bruce Rushton (2008-02-22). "Ax turns out to be Lincoln's last swing". Rockford Register-Star. Retrieved 2009-11-11.
- "Dumas in his Curricle". Blackwood's Edinburgh Magazine. LV (CCCXLI): 351. January–June 1844.
- 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. ISBN 978-3-8424-9722-1.
- Olson, Brad (30 August 2013). "Japan's most sacred site rebuilt, for the 62nd time". CNN News.
- 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.