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Drawings of a ship getting its parts replaced
As the parts of the ship are replaced, the question remains as to whether it is the same ship throughout.

The Ship of Theseus, also known as Theseus's Paradox, is a paradox and common thought experiment about whether an object (in the most common stating of the paradox, a ship) is the same object after having all of its original components replaced with other ones over time, usually one by one.

In Greek mythology, Theseus, the mythical king of the city of Athens, rescued the children of Athens from King Minos after slaying the minotaur and then escaped onto a ship going to Delos. Each year, the Athenians would commemorate this by taking the craft on a pilgrimage to Delos to honour Apollo. Over time, some of its boards rotted and were replaced. A question was raised by ancient philosophers: If no pieces of the original made up the current ship, was it still the Ship of Theseus? If it was no longer the same, when had it ceased existing as the original ship? Seventeenth century philosopher Thomas Hobbes raised the further question of how to consider a second ship that had been built entirely from pieces removed from the original.

In contemporary philosophy, the thought experiment has applications to the philosophical study of identity over time. Within the contemporary philosophy of mind, it has inspired a variety of proposed solutions and concepts regarding the persistence of personal identity.

History

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A fresco from Pompeii depicting Theseus and Ariadne escaping from Crete. According to Plutarch, the Athenians preserved the ship that Theseus used to escape by replacing the parts one by one as they decayed.

In its original formulation, the "Ship of Theseus" paradox concerns a debate over whether a ship that has had all of its components replaced one by one would remain the same ship.[1] The account of the problem has been preserved by Plutarch in his Life of Theseus:[2]

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 strong 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, Life of Theseus 23.1

The seventeenth century philosopher Thomas Hobbes extended the thought experiment by supposing that a ship custodian gathered up all of the decayed parts as they were disposed and used those them to build a second ship, [2] then asked which of the two—the custodian's or the Athenians'—was the "original" ship.[1]

For if that Ship of Theseus (concerning the Difference whereof, made by continual restoration, in taking out the old Planks, and putting in new, the sophisters of Athens were wont to dispute) were, after all the Planks were changed, the same Numerical Ship it was at the beginning; and if some Man had kept the Old Planks as they were taken out, and by putting them afterward together in the same order, had again made a Ship of them, this would, without doubt, had also been the same Numerical Ship with that which was at the beginnings and so there would have been two Ships Numerically the same, which is absurd... But we must consider by what name anything is called when we inquire concerning the Identity of it... so that a Ship, which signifies Matter so figured, will be the same, as long as the Matter remains the same; but if no part of the Matter is the same, then it is Numerically another Ship; and if part of the Matter remains, and part is changed, then the Ship will be partly the same, and partly not the same.

— Hobbes, "Of Identity and Difference"[3]

Hobbes considers the two resulting ships as illustrating two definitions of "Identity" or sameness, form and matter, that are being compared to the original ship:

  1. the ship that maintains the same "form" as the original, that which persists through complete replacement of material and;
  2. the ship made of the same "matter", that which stops being 100 per cent the same ship when the first part is replaced.[3][4]

Proposed resolutions

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The Ship of Theseus paradox can be thought of as an example of a puzzle of material constitution — that is, a problem with determining the relationship between an object and the material of which it is made.[1]

Constitution is not identity

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According to the Stanford Encyclopedia of Philosophy, the most popular solution is to accept the conclusion that the material out of which the ship is made is not the same object as the ship, but that the two objects simply occupy the same space at the same time.[1]

Temporal parts

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Main article: Temporal parts

Another common theory, put forth by twentieth century American philosopher David Lewis, is to divide up all objects into three-dimensional time-slices which are temporally distinct. This avoids the issue that the two different ships exist in the same space at one time and a different space at another time by considering the objects to be distinct from each other at all points in time.[1]

Cognitive science

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According to other scientists, the thought puzzle arises because of extreme externalism: the assumption that what is true in our minds also holds true in the world.[5] Noam Chomsky says that this is not an unassailable assumption, from the perspective of the natural sciences, because human intuition is often mistaken.[6] 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.[7]

Following on from this observation, a significant strand[who?] 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.[8]

Deflationism

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According to the Stanford Encyclopedia of Philosophy, the deflationist view is that the facts of the thought experiment are undisputed; the only dispute is over the meaning of the term "ship" and is thus merely verbal.[1] American philosopher Hilary Putnam asserts that "the logical primitives themselves, and in particular the notions of object and existence, have a multitude of different uses rather than one absolute 'meaning'."[9] This thesis—that there are many meanings for the existential quantifier that are equally natural and equally adequate for describing all the facts—is often referred to as "the doctrine of quantifier variance."[10]

Continued identity theory

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This solution (proposed by Kate, Ernest et al.) sees an object as staying the same as long as it continuously and metaphysically exists under the same identity without being fully transformed at one time. For instance, a house that has its front wall destroyed and replaced at Year One, the ceiling replaced at Year Two, and so on, until every part of the house has been replaced will still be understood as the same house. However, if every wall, the floor, and the roof are destroyed and replaced at the same time, it will be regarded as a new house.[citation needed]

Alternative forms

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In Europe, several independent tales and stories feature knives of which the blades and handles had been replaced several times but are still used and represent the same knife. France has Jeannot's knife,[11][12] Spain uses Jeannot's knife as a proverb, though it is referred to simply as "the family knife", and Hungary has "Lajos Kossuth's pocket knife". Several variants or alternative statements of the underlying problem are known, including the grandfather's axe[13] and Trigger's broom,[14][15] where an old axe or broom has had both its head and its handle replaced, leaving no original components.

The ancient Buddhist text Da zhidu lun contains a similar philosophical puzzle: a story of a traveller 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, and the traveller was confused about who he was.[16]

The French critic and essayist Roland Barthes refers at least twice to a ship that is entirely rebuilt, in the preface to his Essais Critiques (1971) and later in his Roland Barthes par Roland Barthes (1975); in the latter, the persistence of the form of the ship is seen as a key structuralist principle. He calls this ship the Argo, on which Theseus was said to have sailed with Jason; he may have confused the Argo (referred to in passing in Plutarch's Theseus at 19.4) with the ship that sailed from Crete (Theseus, 23.1).

In Japan, the Ise Grand Shrine is rebuilt every twenty years with entirely "new wood". The continuity over the centuries is considered spiritual and comes from the source of the wood, which is harvested from an adjoining forest that is considered sacred.[17][18]

In the United States, a man claims to own the axe that George Washington used to cut down the cherry tree, but the axe's handle has been replaced twice and its head once.[19]

See also

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Citations

[edit]
  1. ^ a b c d e f Wasserman.
  2. ^ a b Blackburn 2016.
  3. ^ a b Hobbes 1656.
  4. ^ Rea 1997, p. xix.
  5. ^ Chomsky 2009, p. 382.
  6. ^ Chomsky 2010, p. 9.
  7. ^ McGilvray 2013, p. 72.
  8. ^ Grand 2003, Introduction.
  9. ^ Putnam, H., 1987, "Truth and Convention: On Davidson's Refutation of Conceptual Relativism", Dialectica, 41: 69–77
  10. ^ Hirsch, E., 1982, The Concept of Identity, Oxford: Oxford University Press. 2002b, "Quantifier Variance and Realism", Philosophical Issues, 12: 51–73.
  11. ^ "Dumas in his Curricle". Blackwood's Edinburgh Magazine. LV (CCCXLI): 351. January–June 1844.
  12. ^ 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.
  13. ^ Browne, Ray Broadus (1982). Objects of Special Devotion: Fetishism in Popular Culture. Popular Press. p. 134. ISBN 0-87972-191-X.
  14. ^ "Heroes and Villains". BBC. Retrieved 16 January 2014.
  15. ^ Casadevall, Nicole; Flossmann, Oliver; Hunt, David (27 April 2017). "Evolution of biological agents: how established drugs can become less safe". BMJ. 357 j1707. doi:10.1136/bmj.j1707. hdl:20.500.11820/807b405b-e5f0-4ca5-95de-056b1fe3f7d7. ISSN 0959-8138. PMID 28450275. S2CID 1826593.
  16. ^ Huang & Ganeri 2021.
  17. ^ 常若（とこわか）＝伊勢神宮・式年遷宮にみる和のサステナビリティ (in Japanese). Daiwa Institute of Research Ltd. 6 April 2016. Archived from the original on 7 May 2021. Retrieved 5 November 2022.
  18. ^ Shinnyo Kawai (2013) 常若の思想 伊勢神宮と日本人. Shodensha. ISBN 978-4396614669
  19. ^ Smith, Nicholas J. J. (March 2008). "Why Sense Cannot Be Made of Vague Identity 1". Noûs. 42 (1): 1–16. doi:10.1111/j.1468-0068.2007.00671.x.

General and cited references

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Ship of Theseus

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The Ship of Theseus is a classic philosophical that explores the nature of identity and persistence through gradual change, questioning whether an object remains the same when all of its original parts are gradually replaced over time, without identifying or privileging any specific part—such as the keel, bow, or another component—as the bearer of the "original identity." The thought experiment concerns the gradual replacement of all parts without designating any single part as essential to identity. In the late first century CE, the Greek biographer described the paradox in his Life of , recounting how the Athenians preserved the thirty-oared on which the mythical had sailed with youths to confront the in and returned safely. They maintained the vessel down to the time of Phalereus (c. 317–307 BCE) by periodically removing decayed timbers and replacing them with new, sound ones, turning it into a tangible example for philosophical debates on growth and identity. As noted: "The ship on which sailed with the youths and returned in safety... was preserved by the Athenians down to the time of Phalereus. They took away the old timbers from time to time, and put new and sound ones in their places, so that the vessel became a standing illustration for the philosophers in the mooted question of growth, some declaring that it remained the same, others that it was not the same vessel." The paradox gained further complexity in the seventeenth century through English philosopher , who in (1655) extended Plutarch's scenario by imagining that the discarded original planks were collected and reassembled into a second ship. This introduces the dilemma of two competing vessels—one continuously repaired with new materials, the other reconstructed from the originals—prompting the question of which, if either, constitutes the true Ship of Theseus. Hobbes used this to illustrate challenges in defining numerical identity for material objects, emphasizing how such replacements disrupt continuity without clear criteria for sameness. Philosophically, the Ship of Theseus underscores enduring issues in metaphysics, particularly the problems of material constitution and diachronic identity—how objects persist or change over time. It challenges intuitions about composition, suggesting that an object might coincide with multiple entities at once or that identity depends on form, function, or historical continuity rather than mere matter. Debates persist without consensus, influencing discussions in , (e.g., human bodies undergoing cellular replacement), and even contemporary fields like , , and blockchain technology, where gradual modifications raise analogous questions about continuity.

Origins and Historical Development

Ancient Greek Roots

The Ship of Theseus originates in mythology surrounding , the legendary founder-king of renowned for his heroic exploits and voyages that unified and established Athenian identity. is depicted as the son of or , embarking on quests such as the journey to where he volunteered to confront the , navigating the with Ariadne's aid and returning triumphantly with the youths he had accompanied. This voyage, detailed in early sources like Bacchylides' Ode 17 from the 5th century BCE, underscores ' role as a civilizing hero who ended the tribute of youths to and symbolized Athenian resilience. The ship used in Theseus' Cretan voyage—a thirty-oared vessel—was preserved as a sacred in ' harbors, serving both commemorative and practical purposes by demonstrating ancient techniques through ongoing maintenance. Historical evidence indicates this relic existed as a tangible artifact in Athenian harbors down to the time of Phalereus (c. 317–307 BCE). Early philosophical roots of identity questions appear in of (c. 535–475 BCE), whose doctrine of emphasized perpetual transformation while preserving underlying unity. In fragment B12, he states: "On those who step into the same rivers, different and again different waters flow," illustrating how entities like rivers maintain sameness amid constant change, a concept echoed in fragment B49a: "We step and do not step into the same rivers; we are and we are not." These ideas prefigure concerns over persistent identity despite flux, influencing later debates on material replacement without altering . Aristotle's (384–322 BCE) treatments in the Categories and Metaphysics offer conceptual precursors through distinctions between substance () and accidents, where a primary substance endures as the underlying subject of changing attributes. In Metaphysics Book Theta, he elaborates potentiality (dynamis) and actuality (energeia) to explain how things persist through change: a seed actualizes into a while retaining its essential form, allowing identity to transcend material alterations. Scholars note these frameworks as early explorations of how composite entities like ships could retain identity amid part replacements, bridging mythology and metaphysics.

Plutarch's Formulation

Plutarch, writing in his Life of Theseus as part of the Parallel Lives around 100 CE, provides the most detailed and influential ancient account of the Ship of Theseus . In Chapter 23, he recounts how the Athenians meticulously maintained the vessel that and the youths of used to sail to and return safely after defeating the . This thirty-oared , a symbol of Theseus's heroic triumph, was preserved as a sacred relic in Athens, with its decaying timbers systematically removed and replaced with new, sound ones over time. The process of replacement unfolded gradually across centuries, beginning in the mythical era of —traditionally placed around 1200 BCE during the —and continuing through the classical and Hellenistic periods. By the time of , the Athenian statesman who served as regent from 317 to 307 BCE, every original plank had been substituted, yet the ship retained its form and function. This ongoing maintenance transformed the ship into a tangible emblem for philosophical debate on identity and change, where some argued it remained numerically the same vessel due to its continuous form, while others contended it was entirely new after total replacement. As describes: "The ship on which sailed with the youths and returned in safety, the thirty-oared , was preserved by the Athenians down to the time of . They took away the old timbers from time to time, and put new and sound ones in their places, so that the vessel became a standing illustration for the philosophers in the mooted question of growth, some declaring that it remained the same, others that it was not the same vessel." Plutarch's version, drawing on earlier Greek traditions, established the as a core in metaphysics, emphasizing the tension between material continuity and formal . The ship's extended into the Roman era, serving as a preserved artifact in Athenian temples and arsenals well into the first century CE, which underscores its cultural significance beyond mere mythology.

Medieval and Renaissance Interpretations

In medieval , the Ship of Theseus paradox found indirect engagement through discussions of substance and accidents, particularly in Thomas Aquinas's (1265–1274), where he explores how a material object's essence persists despite changes in its accidental properties, using examples of composite entities like organic bodies to illustrate the underlying that maintains unity and identity over time. Aquinas's framework, drawing on Aristotelian metaphysics, posits that the ship's form (its structural essence) endures even as parts are replaced, providing a resolution to persistence puzzles akin to Theseus's vessel without directly referencing the ancient tale. This approach bridged with classical , emphasizing divine sustenance of substance amid material flux. During the , the experienced a revival in humanist writings, notably in Michel de Montaigne's Essays (1580), where he analogizes to bodily renewal, questioning whether the self remains the same as physical components change over time, much like a ship rebuilt plank by plank. In essays such as "Of Repentance" and "Of Experience," Montaigne reflects on the body's gradual transformation—evoking ancient notions of septennial renewal—and concludes that identity is fluid yet continuous through the soul's adaptive essence, echoing Plutarch's foundational narrative while applying it to human selfhood. This personalist lens shifted the from collective artifacts to individual introspection, influenced by the era's rediscovery of classical texts. The paradox surfaced in 16th-century debates over relics and artifacts in European churches, where questions of authenticity arose amid Reformation critiques of Catholic veneration, such as John Calvin's Treatise on Relics (1543), which mocked the proliferation of holy fragments like pieces of the , arguing that such multiplications undermine the original object's identity despite claims of preserved sanctity. Similar concerns applied to preserved ecclesiastical items, including model ships or vessels symbolizing apostolic voyages (e.g., relics tied to St. Peter's bark in Roman basilicas), prompting scholastic defenders to invoke to affirm continuity. These controversies highlighted tensions between material alteration and sacred persistence, paralleling Theseus's ship in theological disputes over artifactual identity. With the advent of the printing press in the 15th century, the paradox evolved from mythological anecdote—rooted in Plutarch's Life of Theseus (c. 100 CE)—to abstract metaphysical inquiry, as widespread dissemination of ancient works like Plutarch's Lives fueled humanist analysis and integration into Christian thought. This era marked a transition toward philosophical abstraction, evident in how thinkers like Montaigne abstracted the dilemma into essays on change, paving the way for modern identity debates while retaining its classical core.

Core Paradox and Key Concepts

Statement of the Dilemma

The Ship of Theseus paradox poses the question of whether an object that has all of its components gradually replaced over time remains the same object. Specifically, if every plank, beam, and part of the original ship is substituted with new materials while maintaining its structure and function, does it retain its identity as the original vessel? This dilemma, first narrated in Plutarch's Life of Theseus, challenges the criteria for an object's persistence through change. A complicating variant introduces a second ship constructed from the discarded original parts, reassembled elsewhere, prompting the further inquiry: which of the two ships—the continuously repaired one or the reconstructed one—is the true Ship of Theseus? This dual-ship scenario underscores the tension between numerical identity, which concerns whether two objects are one and the same across time, and qualitative similarity, which involves resemblance in properties without requiring exact sameness. The paradox highlights vagueness in determining the point at which identity ceases, as no precise threshold exists for when sufficient change disrupts continuity. At its core, the puzzle addresses diachronic identity—how an object endures over time—contrasted with synchronic identity, which applies at a single moment. To illustrate, consider a step-by-step replacement: starting with one plank swapped for a new one, the ship intuitively remains the same; after half the parts are replaced, it still seems continuous; yet upon full substitution, doubts arise about its status. This gradual process evokes a sorites-like continuum, where incremental changes accumulate without a clear boundary for loss of identity, amplifying the logical challenge to concepts of persistence. The classic formulation of the paradox does not identify any specific part—such as the keel (the ship's foundational backbone), bow, or another component—as the bearer of the original identity. It questions whether the ship remains the same after all its parts are gradually replaced, focusing on the tension between material composition and continuity of form or function, rather than privileging any single element. While informal discussions sometimes jokingly claim that the keel determines identity, this is not part of the classic philosophical thought experiment.

Questions of Identity and Change

The Ship of Theseus paradox raises fundamental questions about the criteria for an object's identity over time, prompting philosophers to debate whether sameness is determined by material composition, structural form, functional role, or causal-historical continuity. The paradox does not identify any specific part—such as the keel, bow, or another component—as the bearer of the "original identity." Rather, it questions whether the ship remains the same after all its parts are gradually replaced, without privileging any single part. Informal discussions sometimes jokingly claim the keel (the ship's foundational backbone) determines identity, but this is not part of the classic philosophical formulation. Material criteria emphasize the persistence of the same physical parts, such that gradual replacement undermines identity once sufficient components are altered, as seen in atomistic views where objects are bundles of matter. In contrast, form-based criteria prioritize the object's essential structure or design, allowing identity to endure if replacements preserve the original configuration and purpose, regardless of the specific atoms involved. Functional approaches focus on the ship's ongoing role—such as serving as a naval vessel or historical relic—suggesting that identity lies in continued performance of its defining activities. Causal history, meanwhile, traces identity through an unbroken chain of events linking the present object to its origins, where even a fully rebuilt ship might qualify as the same if it inherits the original's spatiotemporal trajectory. These identity criteria intersect with ancient theories of change, particularly the opposition between permanence and Heraclitean , using the ship as a pivotal test case for substance —the study of what constitutes enduring entities. argued for the unchanging unity of being, denying the reality of alteration and asserting that true substances remain identical and eternal, which challenges the ship's persistence amid replacements by implying that any change destroys the original substance. , conversely, embraced as the of reality, where "everything flows" and stability is illusory, allowing the ship to maintain identity through dynamic processes of replacement that reflect the world's constant becoming. In substance , the tests whether entities are primary substances (enduring substrates) or mere aggregates subject to , with the ship's gradual transformation illustrating tensions between static and temporal evolution. The paradox extends to personal identity, analogizing the ship's material flux to the human body's cellular turnover, where most cells are replaced over time, raising doubts about the self's continuity. Scientific estimates indicate that the human body replaces approximately 330 billion cells daily, equivalent to about 1% of total cells, leading to substantial renewal over years, though not all tissues renew at the same rate—erythrocytes every 120 days, but neurons largely persist for life. While the average age of cells in the body is estimated at 7 to 10 years, cell turnover varies widely by tissue, with some cells (such as neurons and certain heart cells) lasting a lifetime and not being replaced, meaning the body does not undergo complete material turnover. This bodily analogy provokes questions of self-identity: if the material basis changes substantially but incompletely, does the person remain the same, or is continuity illusory? John Locke's theory of contrasts memory-based psychological continuity with mere bodily persistence. In his view, consists in the sameness of , where a person at time t2 is identical to one at t1 if they can remember (or are connected through a chain of memories to) the thoughts and actions of the earlier self, independent of bodily changes. Locke explicitly contrasts this with animal or man identity, which relies on bodily continuity, arguing that replacement of human parts analogous to the Ship of Theseus does not disrupt so long as links the stages. Thus, serves as the criterion for the self's endurance amid flux, prioritizing relational history over material or formal fixity.

Distinction Between Material and Form

The hylomorphic theory, central to Aristotle's philosophy, posits that physical substances are composites of matter (hylē) and form (eidos or morphē), where matter provides the underlying substrate and form imparts structure and essence to the whole. In this framework, matter serves as potentiality, capable of receiving different forms and persisting through transformations, while form represents actuality, actualizing the potential of matter into a unified, functional entity. For composite objects like artifacts, this distinction allows the substance to endure changes in its material components without losing its essential identity, as the form maintains the object's organizational integrity and purpose. Applied to the Ship of Theseus paradox, hylomorphism resolves the dilemma by emphasizing that the ship's identity resides in its form—the structural arrangement and functional role as Theseus' vessel—rather than in its specific atomic parts. As planks and timbers are gradually replaced, the matter changes, but the form persists, ensuring the ship remains the same entity so long as its navigational design, historical purpose, and cohesive structure are preserved. This approach shifts focus from material continuity to the enduring actuality of form, which unifies the varying matter into a single, teleologically oriented object. Aristotle developed these ideas in his Physics (circa 350 BCE), where he describes matter as the persistent substrate in processes of change, such as the replacement of parts in a composite substance, and form as the principle that actualizes and defines its kind. In Metaphysics (Book VII), he further elaborates on how , like houses or tools, maintain their substantial unity through alterations, with form ensuring the endurance of the object's essential capacities despite material flux. These 4th-century BCE texts establish as a foundational account of how everyday objects persist amid gradual replacement, influencing later interpretations of identity puzzles. Critiques of this application highlight its limitations, particularly when every element contributing to the form—such as design blueprints or skilled craftsmanship—is also replaced, rendering the distinction between matter and form vague and potentially insufficient for absolute material turnover. In extreme cases, like the paradox's reconstructed duplicate ship, hylomorphism struggles to delineate precise boundaries for form's persistence, leading some scholars to question its adequacy for resolving diachronic identity without additional criteria.

Metaphysical Resolutions

Constitution Versus Identity

In the philosophical resolution to the Ship of Theseus paradox known as the constitution view, an object's identity persists despite gradual replacement of its parts because is a distinct relation from identity itself. At any given time, the ship is constituted by its current planks or s, much like how an object weighs a certain amount without being identical to that weight; this relation allows the ship to survive part replacements while remaining numerically the same . The view emphasizes that strict material identity is not required for , as the ship's formal and functional properties—its structure as a vessel—endure through spatiotemporal continuity of the overall object. This distinction draws on the idea that sortal predicates, such as "ship," identify countable individuals with specific forms, whereas mass terms like "wood" denote undifferentiated matter without implying identity to a particular object. Thus, the original ship at time t1, constituted by initial planks, is not identical to those planks but to the persisting ship-form that later accommodates new materials; dominance of form over matter ensures continuity without violating the paradox's challenge to material sameness. Philosophers like Lynne Rudder Baker argue that this relational constitution avoids the need for part-for-part identity, resolving the dilemma by treating the ship as a unified whole defined relationally across time. Influential discussions, such as Judith Jarvis Thomson's analysis of a statue constituted by clay, illustrate how constituted objects possess unique properties (e.g., aesthetic or navigational form) that their materials lack, reinforcing that constitution is asymmetric and non-transitive with respect to identity. David Lewis's framework in modal realism further supports this by using counterpart relations to explain how the ship could persist in possible worlds involving replacements, prioritizing relational and historical continuity over exact material overlap. This approach has historical precursors in hylomorphic theories, where an object's essence arises from the union of matter and form rather than matter alone. Ultimately, the replaced ship remains the same through its unbroken spatiotemporal history, constituted differently at each stage but identical as the enduring vessel of Theseus.

Perdurantism and Temporal Parts

, also known as , posits that objects through time by having temporal parts, much like they have spatial parts, thereby existing as extended entities in four-dimensional . According to this view, an object is a "space-time worm" composed of a series of temporal stages or parts, each occupying a specific interval of time, rather than being wholly present at every moment of its existence. This theory was notably developed by philosophers such as David Lewis, who defined perdurance as persistence "by having different temporal parts, or stages, at different times, rather like a road having different spatial parts, or segments, in different places," and Theodore Sider, who formalized it in his comprehensive treatment of the of persistence. In applying to the Ship of Theseus paradox, the ship is understood not as a single, unchanging but as the mereological sum of its temporal parts across its lifespan. Each stage of the ship—before and after each plank replacement—constitutes a distinct temporal part related to the others through spatiotemporal continuity and causal connections, preserving overall identity without requiring material sameness. For instance, the early temporal parts of the ship consist of the original planks, while later parts incorporate the replacements, yet the entire four-dimensional ship remains the same object because identity is maintained via parthood relations among these stages, avoiding the need for strict numerical identity at every instant. This framework rejects the problems inherent in endurantism, where objects are said to endure by being wholly present throughout their existence, which struggles to account for qualitative change without invoking ad hoc relations or temporary intrinsics. resolves such issues by explaining change as mere differences between successive temporal parts; for example, the ship's "decaying" or "being repaired" is a variation in the properties of adjacent stages, not a transformation of a single enduring whole. Furthermore, draws support from , which treats time as a akin to in a unified manifold, reinforcing the idea that objects are four-dimensional entities spread out across temporal extents rather than localized punctually in time. This alignment with relativistic physics underscores the theory's compatibility with modern scientific , where persistence is modeled as extension in a block universe containing all temporal parts simultaneously.

Endurantism and Continued Existence

Endurantism posits that ordinary objects, such as the Ship of Theseus, persist through time by being wholly present as three-dimensional entities at every moment of their existence, without being composed of temporal parts. According to this view, change occurs not through the replacement of segments but via the acquisition and loss of accidental properties, while the object's essential nature remains intact. This doctrine aligns with a materialist where objects endure identically across time, confronting the by emphasizing continuity in form or function over material composition. Influential proponents of endurantism include , who in his materialist framework denies the existence of composite artifacts like ships, arguing that only simples and living organisms are real entities; this dissolves the paradox by eliminating the subject of persistence rather than positing a unified substratum for such objects. Similarly, defends endurantism through a hylomorphic approach, using concepts like individual forms and variable embodiment to explain how objects like the ship can persist with changing matter while avoiding coincidence, addressing puzzles such as the Ship of Theseus without invoking temporal parts or vague identity. Drawing from Aristotelian , endurantists often invoke an enduring substratum—comprising matter informed by form—that underlies material flux, ensuring the ship's formal essence (its structure and purpose) persists despite plank substitutions. In resolving the Ship of Theseus dilemma, endurantism maintains the repaired ship's identity through its dominant causal history of continuous use and maintenance, distinguishing it from the reassembled hulk whose discontinuous path severs essential continuity. This approach treats the as a of sortal predicates (e.g., "historical ship of Theseus") that identity to relational properties rather than mere , avoiding strict materialist collapse. Critics of alternative frameworks like highlight their counterintuitiveness, arguing that positing temporal parts multiplies entities unnecessarily and fails to capture the intuitive wholeness of enduring objects through change.

Alternative Variants

The Ship with Both Hulls

In the variant of the Ship of Theseus paradox known as the "two ships" scenario, the original planks discarded during the gradual replacement process are collected and reassembled into a second ship, creating a direct competition between the continuously repaired vessel and the reconstructed original for the title of the true Ship of Theseus. This escalation transforms the dilemma from one of gradual change into a confrontation between two physically distinct objects, each claiming legitimacy through either spatiotemporal continuity or material composition. The logical intensification introduces profound mereological challenges, particularly concerning part-whole relations, as the paradox pits the priority of original materials against the unbroken continuity of form and function in the repaired ship. Philosophers must grapple with whether identity adheres to the sum of parts (favoring the reconstructed ship) or to the persistent structure and historical trajectory (favoring the continuous one), exposing tensions between mereological and the diachronic of objects. Thomas Hobbes addressed this variant in his 1655 work (Part II, Chapter 11), where he illustrates the paradox by noting that gradual replacement of planks preserves numerical identity—"there is no doubt but the old ship was the same numerical ship, both before and after the change"—but reassembling the old planks into a second ship would absurdly yield "two ships numerically the same." Hobbes favors the continuously repaired ship as the authentic one, emphasizing spatial and temporal continuity in the replacement as the key to maintaining identity, rather than mere reconstitution, which disrupts that continuity. This variant's implications undermine simplistic criteria for identity, such as strict (which would privilege the reconstructed ship) or pure formalism (which would support the continuous one), revealing deep ambiguities in assigning to composite objects over time and prompting ongoing in metaphysics about the of numerical sameness.

Modern Analogues in Biology and Technology

In , the Ship of Theseus paradox finds a direct analogue in the continuous turnover of cells and atoms within living organisms. The replaces approximately 330 billion cells each day, accounting for about 1% of its total cellular population of roughly 37 trillion, with short-lived cells like those in the and gut driving much of this renewal. Over longer periods, nearly all atoms are exchanged; tracing radioactive isotopes has shown that 98% of the body's atoms are replaced annually, as metabolic processes incorporate new elements from and air while discarding old ones. This gradual material replacement raises profound questions about organismal identity: if the physical constituents change completely, does the individual remain the same entity, sustained by continuity of form, function, and psychological ? Organ transplants further illustrate this biological parallel, as the introduction of donor tissues—such as a heart or —replaces critical components without disrupting the recipient's sense of self. In extreme cases, like full-face or limb transplants, the integration of foreign biological material challenges notions of bodily continuity, yet legal and social identity remains unchanged, emphasizing relational and psychological criteria over material composition. Philosopher , in his 1984 work , extends these ideas through discussions of fission (as in ) and fusion (as in merging during ), arguing that biological processes undermine strict numerical identity in favor of psychological connectedness and continuity. These scenarios highlight how biological identity endures amid part replacement, much like the reconstructed ship. Post-2010 advancements amplify these concerns. CRISPR-Cas9 gene-editing technology, pioneered in 2012, enables precise DNA modifications that propagate through cellular replication, effectively replacing genetic "parts" across the organism. For instance, therapeutic edits to correct mutations in conditions like alter the heritable blueprint, prompting debates on whether the edited individual shares the same genomic identity as their pre-treatment self. Similarly, advanced prosthetic limbs, such as neural-controlled bionic arms developed since the early , integrate seamlessly with the , extending bodily function into artificial components and blurring the boundary between organic and synthetic identity. These innovations test the by demonstrating how targeted replacements can preserve or enhance continuity without original materials. In technology, analogous processes occur through maintenance and upgrades that sustain object identity despite total part replacement. The U.S. Air Force's B-52 Stratofortress bombers, operational since 1955, have undergone such extensive overhauls that every rivet, wing panel, and engine has been replaced multiple times, yet they retain their model designation and mission role, with plans extending service to 2050. This exemplifies how aircraft identity is defined by design specifications, serial numbers, and operational history rather than original hardware. Software systems provide another parallel: through iterative updates, codebases are incrementally rewritten—often replacing 100% of the source over years—while preserving the application's core functionality, , and versioning identity, as seen in long-lived platforms like operating systems. Nanotechnology introduces self-repairing concepts that mimic biological renewal on a microscopic scale. Self-healing , such as polymers infused with nanocapsules that release repair agents upon damage, autonomously restore integrity at the atomic level without external intervention. Developed since the early 2000s and refined in applications like coatings, these systems challenge the by enabling objects to maintain form through ongoing, internal part replacement, akin to cellular regeneration but engineered for durability. Such technologies underscore a shift toward dynamic identity in machines, where persistence relies on adaptive structures rather than static components. In blockchain technology and cryptocurrencies, the paradox applies to protocol upgrades and hard forks, where changes to the network rules can lead to chain splits and debates over continuity. A key example is the 2016 hard fork on Ethereum following the exploitation of The DAO smart contract, which created two chains: Ethereum (ETH), which implemented the change to reverse the stolen funds, and Ethereum Classic (ETC), which preserved the original unaltered ledger. This division prompted questions about which chain constituted the authentic Ethereum, with arguments focusing on immutability, community consensus, and the original social contract. Similarly, the 2017 Bitcoin hard fork over block size disagreements produced Bitcoin Cash (BCH) alongside the continuing Bitcoin (BTC) chain, raising analogous issues of identity in decentralized systems lacking central authority. These cases highlight how digital identity in leaderless networks is often resolved through market adoption, economic value, and social consensus rather than unbroken historical continuity or unchanged code.

Analogues in Other Philosophical Puzzles

The paradox of the and the lump of clay exemplifies a key analogue to the Ship of Theseus in debates over material constitution, where a single lump of clay is molded into a , raising the question of whether the and the lump are numerically identical despite occupying the same spatiotemporal location but possessing different modal properties, such as the statue's ability to survive reshaping while the lump cannot. This puzzle underscores the distinction between an object's matter and its form or structure, much like the replacement of planks in Theseus's ship challenges whether the resulting vessel retains its original identity. Another related thought experiment is the case of the ripening banana, which gradually changes from green to yellow over time, prompting inquiries into the persistence of identity amid continuous alteration: at what point does the banana cease to be the same object as its earlier stage, paralleling the incremental substitutions in the Ship of Theseus that blur boundaries of sameness? Philosophers like Ryan Wasserman use this example to illustrate the problem of change, where an object's diachronic identity seems preserved despite qualitative shifts, highlighting tensions in mereological and temporal persistence theories. The of serves as a logical kin to the Ship of Theseus, both involving sorites-like series of gradual modifications that undermine sharp identity criteria; in the standard Sorites, adding grains to form a heap raises borderline cases, while non-standard variants explicitly model the Ship's plank replacements as a chain where each step preserves identity yet the whole process seemingly destroys it. This structural affinity, as analyzed by , positions the Ship within broader puzzles of fuzzy or indeterminate identity, where vague predicates complicate diachronic sameness. David Wiggins, in his seminal work Sameness and Substance, links the Ship of Theseus to distinctions between artifactual and identities, arguing that the ship's depends on sortal concepts like "ship" that incorporate principles of unity and continuity, rather than mere material composition, thereby resolving identity questions through absolute, non-relativized sameness. Wiggins contrasts this with natural substances, where identity tracks essential properties independent of human-imposed forms, using the Ship to critique relativist views of identity. Influenced by David Hume's bundle theory of objects, which posits that entities are mere collections of properties without underlying substrata, the Ship of Theseus illustrates how perceived identity arises from resemblance and causation among changing parts rather than any enduring essence; Hume himself invokes the Ship in A Treatise of Human Nature to argue that such continuity is a fiction of the imagination. This Humean perspective frames the paradox as evidence against substantial views of identity, emphasizing relational bundles over fixed substances. The Ship of Theseus illuminates fission cases in , such as the formulated by , where a person is scanned and reconstructed at a distant location after original destruction, or split into two copies, mirroring the dual-ship variant and questioning whether either result preserves numerical identity or merely psychological continuity. Parfit uses this to argue that personal identity is not what matters in survival, akin to how Theseus's puzzle decenters strict continuity in favor of relational or functional criteria. For brief illustration, biological processes like cellular turnover in organisms echo these issues, where gradual replacement sustains apparent sameness without preserving all original matter.

Modern Applications and Interpretations

Cognitive and Psychological Perspectives

Psychological research on identity persistence, inspired by paradoxes like the Ship of Theseus, has explored how humans, from infancy, rely on spatiotemporal continuity to track object identity. In seminal experiments, 10-month-old infants demonstrated the ability to individuate objects based on their trajectories across occluders, using spatiotemporal cues rather than featural properties alone, suggesting an early bias toward continuity in space and time for maintaining numerical identity. This foundational work indicates that even preverbal children prioritize causal and spatiotemporal histories over mere resemblance in judging whether an object remains the same entity, laying the groundwork for adult intuitions about persistence through gradual replacement. Neuroscience studies further reveal cognitive biases favoring causal history in identity judgments, with showing involvement of the ventral visual stream in processing object continuity. Functional MRI (fMRI) demonstrates that regions in the ventral cortex, such as the lateral occipital complex, encode spatiotemporal of objects, enabling the to represent entities as enduring despite changes in appearance or location. surveys post-2010 have critiqued folk metaphysics by testing intuitions on Ship of Theseus scenarios, finding that across cultures often split their judgments: attributing to the ship with continuous form (gradual plank replacement) but denying it to the reassembled original due to disrupted spatiotemporal history, highlighting a preference for causal continuity over resemblance. These biases underscore how everyday identity ascriptions blend historical tracking with structural similarity. Key theories in , such as Daniel Dennett's of , extend to self-identity by positing the self as a dynamic constructed from distributed brain processes rather than a fixed entity, implying resilience to part replacement akin to the Ship of Theseus as long as coherence persists. Evidence from patients supports notions of fragmented identity, where leads to hemispheres operating semi-independently, with conflicting behaviors and beliefs emerging, as seen in cases where one hand acts contrary to the other's intentions, challenging unified self-persistence.

Implications in Ethics and Law

The Ship of Theseus paradox raises profound questions in concerning amid bodily changes, such as organ transplants, where recipients may undergo extensive physiological replacements over time. Philosophers like argued that persists through continuity of and memory, rather than unaltered material substance, allowing a person to remain the same moral agent despite such transformations. This perspective informs ethical debates on post-transplant , , and , as the paradox challenges whether incremental bodily alterations disrupt one's forensic responsibility for actions. Locke's emphasis on as the basis of identity also influences contractual ethics, where ongoing personal continuity underpins obligations in agreements, even as circumstances evolve. In legal contexts, the paradox illuminates following mergers, particularly through the doctrine of successor in U.S. law, which holds acquiring entities accountable for predecessors' debts when substantial continuity exists despite asset reallocations. Courts invoke tests like de facto merger to assess if the successor effectively "replaces" the original without breaking identity, ensuring protection for stakeholders akin to preserving the ship's historical essence. Similarly, salvage rights for shipwrecks under admiralty law hinge on establishing the vessel's historical identity and continuity, determining and recovery claims even after degradation or partial recovery. Contemporary applications extend to 21st-century challenges like technology, which fabricates synthetic personas that erode in by mimicking individuals without consent. A 2025 study leverages the Ship of Theseus paradox to critique regulatory frameworks for deepfakes in political advertising, arguing that just as replaced planks question a ship's sameness, altered digital likenesses demand redefined protections for authentic selfhood. International treaties, such as the UNESCO 1970 Convention on the Means of Prohibiting and Preventing the Illicit Import, Export and Transfer of Ownership of , address artifact preservation by prioritizing historical continuity over material integrity, mirroring conservation practices where partial replacements maintain authenticity. In AI ethics during the 2020s, the applies to machine upgrades, where iterative enhancements—such as hardware swaps or algorithmic retraining—prompt inquiries into whether the resulting system retains the original entity's ethical identity and agency. Scholars contend that these transformations parallel the ship's plank replacements, necessitating frameworks to evaluate continuity for in autonomous systems. This has spurred discussions on in AI deployment, emphasizing relational and functional over static components.

Cultural and Literary References

The paradox has profoundly influenced , , television, and other media, serving as a for questions of identity, continuity, and transformation. In , and Doug Dorst's experimental 2013 S. is structured around a fictional book titled Ship of Theseus by the enigmatic V.M. Straka, using and inserted materials to mirror the paradox's themes of authenticity, replacement, and layered identities. Anand Gandhi's 2012 anthology film Ship of Theseus explicitly adopts the as its title and conceptual framework, weaving three interconnected stories that probe identity through physical transplants, spiritual beliefs, and ethical dilemmas in modern . Christopher Nolan's 2006 film invokes the through its narrative of rival magicians employing cloning technology, raising ambiguities about which duplicated version constitutes the "true" amid obsession and . The HBO series Westworld (2016–2022) frequently references the paradox to explore the nature of consciousness in android "hosts," whose bodies are iteratively rebuilt and memories altered, blurring lines between original and reconstructed entities. In video games, the open-source PC port Ship of Harkinian (2020–present) of The Legend of Zelda: Ocarina of Time derives its name from the paradox, symbolizing the project's piecemeal recreation and enhancement of Nintendo's 1998 classic while preserving its core essence. Ted Chiang's 2010 novella applies the paradox to , depicting "digients" (digital entities) that evolve through upgrades and copies, prompting reflections on whether enhanced versions retain their original and .

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