The Ship of Theseus, also known as Theseus’s Paradox, is a thought experiment and paradox about whether an object is the same object after having had all of its original components replaced over time, typically one after the other.

In Greek mythology, Theseus, mythical king and founder of the city 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 ship on a pilgrimage to Delos to honour Apollo. A question was raised by ancient philosophers: After several hundreds of years of maintenance, if each individual piece of the Ship of Theseus was replaced, one after the other, was it still the same ship?

In contemporary philosophy, this thought experiment has applications to the philosophical study of identity over time, and has inspired a variety of proposed solutions and concepts in contemporary philosophy of mind concerned with the persistence of personal identity.

In its original formulation, the “Ship of Theseus” paradox concerns a debate over whether or not a ship that 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 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, Life of Theseus 23.1

Over a millennium later, the philosopher Thomas Hobbes extended the thought experiment by supposing that a ship custodian gathered up all of the decayed parts of the ship as they were disposed of and replaced by the Athenians, and used those decaying planks to build a second ship.^[2] Hobbes posed the question of which of the two resulting ships, the custodian’s or the Athenians’, was the same ship as the “original” ship.^[1]

For if that Ship of Theseus (concerning the Difference whereof, made by continual reparation, 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, 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 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[edit]

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[edit]

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[edit]

Main article: Temporal parts

Another common theory put forth by David Lewis, is to divide up all objects into three-dimensional time-slices which are temporally distinct, which 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[edit]

According to other scientists, the thought puzzle arises because of extreme externalism: the assumption that what is true in our minds is 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[edit]

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[edit]

This solution (proposed by Kate, Ernest et al.) sees an object as staying the same as long as it continuously 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 1, the ceiling replaced at year 2, 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 known as a new house.^{[citation needed]}

Alternative forms[edit]

In Europe, several independent tales and stories feature knives that have had their blades and handles 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 Tin Woodman, a character in the fictional Land of Oz, was originally a man of flesh and blood, but all his body parts were replaced one by one by metal parts. nevertheless, he retains his identity.

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]

Theseus (UK: /ˈθiːsjuːs/, US: /ˈθiːsiəs/; Greek: Θησεύς [tʰɛːsěu̯s]) was a divine hero and the founder of Athens from Greek mythology. The myths surrounding Theseus, his journeys, exploits, and friends, have provided material for storytelling throughout the ages.

Theseus is sometimes described as the son of Aegeus, King of Athens, and sometimes as the son of the god Poseidon. He was raised by his mother, Aethra, and, upon discovering his connection to Aegeus, travels overland to Athens, having many adventures on the way. When he reaches Athens, he finds that Aegeus is married to Medea (formerly wife of Jason), who plots against him.

The most famous legend about Theseus is his slaying of the Minotaur, half man and half bull. He then goes on to unite Attica under Athenian rule: the synoikismos (‘dwelling together’). As the unifying king, he is credited with building a palace on the fortress of the Acropolis. Pausanias reports that after synoikismos, Theseus established a cult of Aphrodite (‘Aphrodite of all the People’) on the southern slope of the Acropolis.

Plutarch‘s Life of Theseus makes use of varying accounts of the death of the Minotaur, Theseus’s escape, and his romantic involvement with and betrayal of Ariadne, daughter of King Minos.^[i]

Plutarch’s avowed purpose is to construct a life that parallels the Life of Romulus, the founding myth of Rome. Plutarch’s sources, not all of whose texts have survived independently, include Pherecydes (mid-fifth century BC), Demon (c. 400 BC), Philochorus, and Cleidemus (both fourth century BC).^[1] As the subject of myth, the existence of Theseus as a real person has not been proven, but scholars believe that he may have been alive during the Late Bronze Age,^[2] or possibly as a king in the 8th or 9th century BC.^[3]

Aegeus, one of the primordial kings of Athens, was childless. Desiring an heir, he asked the Oracle of Delphi for advice. Her cryptic words were “Do not loosen the bulging mouth of the wineskin until you have reached the height of Athens, lest you die of grief.” Aegeus did not understand the prophecy and was disappointed. He asked the advice of his host Pittheus, king of Troezen. Pittheus understood the prophecy, got Aegeus drunk, and gave Aegeus his daughter Aethra.^[4]

But following the instructions of Athena in a dream, Aethra left the sleeping Aegeus and waded across to the island of Sphairia that lay close to Troezen’s shore. There, she poured a libation to Sphairos (Pelops’s charioteer) and Poseidon and was possessed by the sea god in the night. The mix gave Theseus a combination of divine as well as mortal characteristics in his nature; such double paternity, with one immortal and one mortal, was a familiar feature of other Greek heroes. After Aethra became pregnant, Aegeus decided to return to Athens. Before leaving, however, he buried his sandals and sword under a huge rock^[ii] and told Aethra that when their son grew up, he should move the rock, if he were heroic enough, and take the tokens for himself as evidence of his royal parentage. In Athens, Aegeus was joined by Medea, who had left Corinth after slaughtering the children she had borne to Jason, and had taken Aegeus as her new consort.

Thus Theseus was raised in his mother’s land. When Theseus grew up to be a young man, he moved the rock and recovered his father’s tokens. His mother then told him the truth about his father’s identity and that he must take the sword and sandals back to the king Aegeus to claim his birthright. To journey to Athens, Theseus could choose to go by sea (which was the safe way) or by land, following a dangerous path around the Saronic Gulf, where he would encounter a string of six entrances to the Underworld,^[iii] each guarded by a chthonic enemy. Young, brave, and ambitious, Theseus decided to go alone by the land route and defeated many bandits along the way.

The six entrances to the underworld, more commonly known as the Six Labours, are as follows:

• At the first site, which was Epidaurus, sacred to Apollo and the healer Asclepius, Theseus turned the tables on the chthonic bandit, Periphetes, the Club Bearer, who beat his opponents into the Earth, taking from him the stout staff that often identifies Theseus in vase-paintings.
• At the Isthmian entrance to the Underworld was a robber named Sinis, often called “Pityokamptes” (Greek: Πιτυοκάμπτης, lit. “he who bends Pinetrees”). He would capture travelers, tie them between two pine trees that were bent down to the ground, and then let the trees go, tearing his victims apart. Theseus slew him by his own method. He then seduced Sinis’s daughter, Perigune, fathering the child Melanippus.
• In another deed north of the Isthmus, at a place called Crommyon, he killed an enormous pig, the Crommyonian Sow, bred by an old crone named Phaea. Some versions name the sow herself as Phaea. The Bibliotheca by Pseudo-Apollodorus described the Crommyonian Sow as an offspring of Typhon and Echidna.
• Near Megara, a robber named Sciron forced travelers along the narrow cliff-face pathway to wash his feet. While they knelt, he kicked them off the cliff behind them, where they were eaten by a giant turtle (or, in some versions, a sea monster). Theseus pushed him off the cliff where he was eaten as well.
• Another of these enemies was Cercyon, king at the holy site of Eleusis, who challenged passers-by to a wrestling match and, when he had beaten them, killed them. Theseus beat Cercyon at wrestling and then killed him instead.
• The last bandit was Procrustes the Stretcher, who had two beds, one of which he offered to passers-by in the plain of Eleusis. He then made them fit into it, either by stretching them or by cutting off their feet. Since he had two beds of different lengths, no one would fit. Theseus once again employed Procrustes’ own method on him, cutting off his legs and decapitating him with his axe.

Medea, the Marathonian Bull, Androgeus, and the Pallantides

When Theseus arrived in Athens, he did not reveal his true identity immediately. Aegeus gave him hospitality but was suspicious of the young, powerful stranger’s intentions. Aegeus’s consort Medea recognized Theseus immediately as Aegeus’ son and worried that Theseus would be chosen as heir to Aegeus’ kingdom instead of her son Medus. She tried to arrange to have Theseus killed by asking him to capture the Marathonian Bull, an emblem of Cretan power.

On the way to Marathon, Theseus took shelter from a storm in the hut of an ancient woman named Hecale. She swore to make a sacrifice to Zeus if Theseus were successful in capturing the bull. Theseus did capture the bull, but when he returned to Hecale’s hut, she was dead. In her honor, Theseus gave her name to one of the demes of Attica, making its inhabitants in a sense her adopted children.

When Theseus returned victorious to Athens, where he sacrificed the Bull, Medea tried to poison him. At the last second, Aegeus recognized the sandals and the sword and knocked the poisoned wine cup from Theseus’s hands. Thus father and son were reunited, and Medea fled to Asia.^[5]

When Theseus appeared in the town, his reputation had preceded him, as a result of his having traveled along the notorious coastal road from Troezen and slain some of the most feared bandits there. It was not long before the Pallantides‘ hopes of succeeding the childless Aegeus would be lost if they did not get rid of Theseus (the Pallantides were the sons of Pallas and nephews of King Aegeus, who was then living at the royal court in the sanctuary of Delphic Apollo).^[6] So they set a trap for him. One band of them would march on the town from one side while another lay in wait near a place called Gargettus in ambush. The plan was that after Theseus, Aegeus, and the palace guards had been forced out the front, the other half would surprise them from behind. However, Theseus was not fooled. Informed of the plan by a herald named Leos, he crept out of the city at midnight and surprised the Pallantides. “Theseus then fell suddenly upon the party lying in ambush, and slew them all. Thereupon the party with Pallas dispersed,” Plutarch reported.^[7]

Pasiphaë, wife of King Minos of Crete, had several children. The eldest of these, Androgeus, set sail for Athens to take part in the Panathenaic Games, which were held there every four years. Being strong and skillful, he did very well, winning some events outright. He soon became a crowd favorite, much to the resentment of the Pallantides, who assassinated him, incurring the wrath of Minos.

When King Minos heard what had befallen his son, he ordered the Cretan fleet to set sail for Athens. Minos asked Aegeus for his son’s assassins, saying that if they were to be handed to him, the city would be spared. However, not knowing who the assassins were, King Aegeus surrendered the whole city to Minos’ mercy. His retribution was to stipulate that at the end of every Great Year, which occurred after every seven cycles on the solar calendar, the seven most courageous youths and the seven most beautiful maidens were to board a boat and be sent as tribute to Crete, never to be seen again.

In another version, King Minos had waged war with the Athenians and was successful. He then demanded that, at nine-year intervals, seven Athenian boys and seven Athenian girls were to be sent to Crete to be devoured by the Minotaur, a half-man, half-bull monster that lived in the Labyrinth created by Daedalus.

On the third occasion, Theseus volunteered to talk to the monster to stop this horror. He took the place of one of the youths and set off with a black sail, promising to his father, Aegeus, that if successful he would return with a white sail.^[iv] Like the others, Theseus was stripped of his weapons when they sailed. On his arrival in Crete, Ariadne, King Minos’ daughter, fell in love with Theseus and, on the advice of Daedalus, gave him a ball of thread (a clew), so he could find his way out of the Labyrinth.^[v] That night, Ariadne escorted Theseus to the Labyrinth, and Theseus promised that if he returned from the Labyrinth he would take Ariadne with him. As soon as Theseus entered the Labyrinth, he tied one end of the ball of string to the doorpost and brandished his sword which he had kept hidden from the guards inside his tunic. Theseus followed Daedalus’ instructions given to Ariadne: go forwards, always down, and never left or right. Theseus came to the heart of the Labyrinth and upon the sleeping Minotaur. The beast awoke and a tremendous fight occurred. Theseus overpowered the Minotaur with his strength and stabbed the beast in the throat with his sword (according to one scholium on Pindar’s Fifth Nemean Ode, Theseus strangled it).^[8]

After decapitating the beast, Theseus used the string to escape the Labyrinth and managed to escape with all of the young Athenians and Ariadne as well as her younger sister Phaedra. Then he and the rest of the crew fell asleep on the beach of the island of Naxos, where they stopped on their way back, looking for water. Theseus then abandoned Ariadne, where Dionysus eventually found and married her. On his way back from Crete, he also stopped on the island of Delos, where, according to Plutarch, “Theseus danced with the young Athenians a dance still performed by the inhabitants of the island, consisting of twisting and twisted movements that reproduce the shapes of the labyrinth. Dicearchos states that this dance is called ‘Crane’.”^[9] Theseus forgot to put up the white sails instead of the black ones, so his father, the king, believing he was dead, died by suicide, throwing himself off a cliff of Sounion and into the sea, causing this body of water to be named the Aegean Sea.

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