theseus ship

Unit 2: Metaphysics

Ship of Theseus

The  ship of Theseus , also known as  Theseus’ 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.

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

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.

Modern variations

John Locke proposed a scenario 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.

The French equivalent is the story of Jeannot’s knife, where the eponymous knife has had its blade changed fifteen times and its handle fifteen times, but is still the same knife. In some Spanish-speaking countries, Jeannot’s knife is present as a proverb, though referred to simply as “the family knife”. The principle, however, remains the same.

A Hungarian version of the story features “Lajos Kossuth’s pocket knife”, having its blade and handle continuously replaced but still being referred to as the very knife of the famous statesman. As a proverbial expression it is used for objects or solutions being repeatedly renewed and gradually replaced to an extent that it has no original parts.

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.

Examples 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.

Examples in Japan

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. The shrine has currently been rebuilt 62 times.

Proposed resolutions

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”.

Aristotle’s causes

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 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, “ gleich ” (“equal”) and “ selbe ” (“self-same”) are the pertinent terms, respectively. At least in formal speech, the former refers to qualitative identity (e.g.  die gleiche Murmel , “the same [qualitative] marble”) and the latter to numerical identity (e.g.  die selbe Murmel , “the same [numerical] marble”). Colloquially, “ gleich ” is also used in place of “ selbe “, however.

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, each time-slice and 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.

Citation and Use

Levin, Noah, ed. “Introduction to Philosophy and the Ship of Theseus.” In Ancient philosophy reader, AN OPEN EDUCATIONAL RESOURCE . NGE Far Press, 2019. https://human.libretexts.org/Bookshelves/Philosophy/Book%3A_Ancient_Philosophy_Reader_(Levin)/01%3A_The_Start_of_Western_Philosophy_and_the_Pre-Socratics/1.01%3A_Introduction_to_Philosophy_and_the_Ship_of_Theseus

Use of this work is governed by CC-BY-SA-NC license.

Ship of Theseus Copyright © 2020 by Noah Levin is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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The Ship of Theseus

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From The If Machine by Peter Worley

When doing The Ship of Theseus chapter from The If Machine make the puzzle visual by assembling a model of the ship on the floor with some identical pencils and a piece of paper for the sail. As you explain the process of gradual change replace each pencil with another pencil creating a pile of the discarded pencils to one side of the ship. Invite the group to come up and use the model to demonstrate their ideas for the benefit of everyone in the group.

This extension to the session already in the book helps to make the exercise visual and kinesthetic so that other learning styles in addition to the auditory can be accommodated.

This example was famously supplied as a philosophical thought experiment by Thomas Hobbes, the British philosopher of the English civil war, into which he was born (he said that he was born twins with fear), but he drew the example from Plutarch, a Roman writer, and, of course, Theseus comes from Greek mythology.

In order for this session to be philosophically fruitful it is necessary to understand the philosophical subtleties involved in an exploration of the thought experiment. The embedded question to bear in mind in this whole discussion is if it is a new ship when all the parts are replaced then at what point does it become a new ship? This is where a lot of the philosophy will lie because here we are faced with the ‘problem of vagueness’.

If it is a new ship when the parts are all replaced and only then, would that mean that when it only had one part left to replace, it was still the old ship? If so, this seems a little odd. If not, then when does it become the new ship? This particular problem is known as the sorities paradox (from the Greek word for ‘heap’): how many grains of sand make a heap?

For the purposes of keeping things clear for the discussion it is a good idea to illustrate all this with examples as you go. I would ask the children to imagine that the ship has 100 parts. You can then use this at each point of the discussion to have them explore and reach the difficulties for themselves. For example, if someone says that it would be the new ship when more than half the parts are replaced, test it: so are you saying that it is a new ship when it has 51 metal parts and only 49 wooden ones? Then ask everyone what they think about this.

Theseus owned a ship and the ship was entirely made of wood. Every time a piece of the ship needed replacing it was replaced with a metal part. This went on for a few years until eventually it was entirely made of metal.

Task Question

• Is the metal ship of Theseus the same ship as the wooden ship of Theseus?

It may be helpful to draw a diagram similar to this as you explain the scenario:

There is very likely to be materialists aboard your classroom, that is to say, those that will maintain that the ship is different if it is made of different stuff (“it was made of wood but now it’s made of metal!”) Thomas Hobbes was also a materialist in his response to the problem. He proposed a version of the following thought experiment to anyone who would entertain that the metal ship is the same as the wooden ship:

On board the ship was a sailor who really wanted his own ship but could not afford one. So, he came up with a plan: every time Theseus decided to replace one of the wooden parts of his ship with a metal part the sailor would take the discarded piece of wood and hide it in his shed. When eventually he had collected all the wooden parts he re-assembled them into a ship again.

• Does this mean there are two ships of Theseus or one? (Again, use diagrams to explain all this)

The sorts of ideas you might expect to hear in this session are as follows (often in different words):

* That the ship is different the moment the first plank is replaced because any change would result in a different ship . Later, when you move to a discussion of personhood, a response question to this point could be: does that mean that any change to myself/yourself, such as a tooth falling out, makes me/you a different person?

* That the ship is different only when the last piece has been replaced because only then is there none of the original ship left . Response question: does that mean that when there is only one piece of the original ship left it is still the old ship?

* If it suddenly changed into metal then it would be different, but if it changes gradually then it is the same ship because, at each stage of change, it is related to the old ship in that it is only minutely different.

* Even though the materials it’s made of change, the shape, the name and the design stay the same, so it is the same ship.

* It is the same ship if other people think it’s the same ship.

The Self of Theseus

At some point you will want to talk about how the discussion of the ship pertains to how we think of ourselves. This will either happen very naturally when the children start to make the connection, or, you will need to make the connection explicit yourself. Here are some suggestions of how it can be done:

Show two photographs next to each other of a person as a young child and as an old person. Ask the children whether they think they are the same person and why.

If you are with older children (age 10 upwards) then you can explain how scientists tell us that our cells are completely replaced every 7 years or so and then ask the children if this means that they are a different person every 7 years.

Nested Questions 

• What is it that makes us the same person through time?

Possible responses to this problem are as follows:

* People and things are different . Response question: how are they different? * People have thoughts and memories but ships don’t. * We might change on the outside but our personalities stay the same. Response question: does that mean that our personalities can’t change?

Each of these insights can lead to further related discussions in themselves.

For the philosopher John Locke (1632-1704) it is that we are linked by memory to our past selves that makes us the same person through time. So, for Locke, it is not our body that makes us the same person – as this is constantly changing – but our mental life that makes us a person that lasts through change. Response question: if we lose our memory would that make us a different person?

Online philosophy

Vagueness Sorities paradox

Concrete and personalised discussions

When doing philosophy with children they may begin to lose interest if the discussions remain too abstract or irrelevant to their own experience, so it is often good to begin with concrete examples, such as – in this chapter – a scenario with a ship where the philosophical problems always have a concrete reference for the children to test them out. Also, making the discussions about the children in some way can keep them engaged with the issues. Putting the insights and ideas about the ship into the context of their own lives and experience can bring it vividly to life for them. Many of the children will have been thinking along these lines already and it can be reassuring for those children to find that there is a whole tradition of thinking about these ideas stretching back many hundreds – in some cases thousands – of years.

See also chapter…on memory loss

Download The Ship of Theseus

Ages: Ages 7-11 (KS2)

Subjects: Metaphysics

Themes: Vagueness , Sorites paradox , Personal identity , Change

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The Ship Of Theseus Thought Experiment

The Ship of Theseus, otherwise known as Theseus’ Paradox, is a thought experiment that continues to be a popular topic in philosophy today.

The Ship of Theseus, also known as Theseus’ Paradox, is a fascinating thought experiment that has intrigued scholars for centuries. It raises thought-provoking questions about the concept of identity over time. Imagine a scenario where each part of a ship is gradually replaced, one at a time. The fundamental inquiry emerges: Is the vessel that remains after all the replacements the same ship as the one that existed before? Delve into this ancient puzzle and explore the enduring debates it continues to spark.

Ship of Theseus: The Myth Behind the Paradox

To begin with, it may be of interest to explore the myth behind the Ship of Theseus paradox.

Theseus was a young prince of Athens in Ancient Greece . He was raised away from the kingdom by his mother, Aethra. Upon coming of age, he was told of his true identity as heir to the Athenian throne, and so he set out to claim his birth-right. Reaching Athens, he wanted to find ways of proving his worthiness of succeeding to the throne. To his dismay, he found that the King of Athens , Aegeus, was paying a terrible tribute to the King of Crete, King Minos because he had lost a war to Minos previously.

The tribute was seven girls and seven boys, who were given up to King Minos, to be put in a dangerous Labyrinth, impossible to navigate, and roamed by a ferocious monster, the Minotaur. The Minotaur was a half-man, half-bull, a mythical creature that would devour the boys and girls. Theseus volunteered as tribute to be among the seven boys who were given up to King Minos each year. Theseus had big plans; he wanted to kill the Minotaur, save the children, and stop the tribute.

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Here comes the first instance of the ship. King Aegeus was very sad about his son, Theseus, setting sail to potential death, so Theseus promised his father that if he should return, the ship would show white sails. If he perished, the sails would show their normal color, black.

The Ship Of Theseus: Adventures In The Aegean

Theseus and the other girls and boys set sail to Crete on their ship, which would be known as the Ship of Theseus. They disembarked at Crete and held an audience with the royal family. Here is where Theseus met Ariadne, the princess of Crete, and the two fell madly in love.

In a secret meeting before entering the maze, Ariadne slipped a ball of thread and a sword to Theseus. He used these gifts to escape, using the sword to kill the Minotaur, and the string to guide himself back out of the maze. Theseus, the other tributes, and Ariadne snuck back onto the ship and set sail to Athens before King Minos could figure out what they had done.

Along the way, the ship of Theseus stopped at the island of Naxos. Here, the story varies in many versions, but Ariadne was left behind, and Theseus left for Athens without her. Ariadne later married the god Dionysus . In distress or ignorance, Theseus then forgot to change the color of the sail, so it remained black. Upon seeing the black sails, King Aegeus was deeply distraught and threw himself from a cliff into the Aegean waters below.

Theseus disembarked from the ship and heard the news of his father’s death. He was very upset but took on the mantle to be the next King of Athens. Then, according to Plutarch, the Ship of Theseus was stored in a museum in Athens, to be a reminder of Theseus’ miraculous feats, and the tragedy of King Aegeus.

Ship Of Theseus: The Question

Many philosophers, including Heraclitus and Plato , deliberated on the paradox. Plutarch, a biographer, philosopher, and social historian from the 1st century A.D. mentions the paradox of Theseus’ Ship, in his work, the Life of Theseus :

“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, 1st — 2nd century CE)

The paradox is that if the Athenians replaced each plank of the ship with a new piece of wood every time it began to rot, there would eventually come a time when all planks were replaced, and no plank would be from the original ship. Does this mean that the Athenians still have the same ship as Theseus?

Plutarch uses a ship analogy, but the concept applies to any object. If, over time, each component of a thing is replaced, is the object still the same? If not, when did it cease to be itself?

The Ship of Theseus thought experiment has held a strong place in identity metaphysics and calls into question the boundaries and flexibility of identity. Many think that the experiment has no answers, but others have attempted to find a resolution. By considering the ways in which the experiment has been applied, we can gain a better understanding of the Ship of Theseus.

The Living And The Inanimate

The experiment applies not just to inanimate objects like the ‘ship’, but to living beings, too. Consider having two photos side by side of the same person, one picture shows the person in old age and the other picture shows the person in their youth. The experiment asks, how is the person in the two pictures the same, and how are they different?

The body continually regenerates cells, and science tells us that after seven years, the entire body no longer has any of its original cells. Therefore, the human body, just like the Ship of Theseus, has come to be different to its original form, because the old parts have been replaced with new ones to create an entirely new object.

Heraclitus , quoted by Plato in the Cratylus , argued that “all things move and nothing remains still” . This argument maintains that nothing retains its identity, or that identity is a fluid concept, and never one thing for very long. Therefore, neither ship is the original Ship of Theseus.

Regarding the above example, some theorists argue that objects like the ship, are different to a human being because a human has memories, whereas an inanimate object, does not. This comes from John Locke ’s theory that it is our memory that links us through time to our past selves.

Therefore, is identity tied to memory, body, neither, or a combination of the two?

Thomas Hobbes & Transitivity Theory

Thomas Hobbes steered the Ship of Theseus discussion in a new direction by asking what would happen if after the original material (the rotten planks of the ship) had been discarded, they were collected and reassembled to build a second ship? Would this new, second ship, be the original ship of Theseus, or would the other ship that had been repeatedly fixed still be the Ship of Theseus? Or neither, or both?

This brings us to the theory of transitivity . The theory states that if A = B, and B = C, this means that A must = C. Putting this into practice:  Theseus’ original ship, just harbored, is A. The ship with all the new parts is B. The re-constructed ship is C.  By the law of transitivity, this would mean that all ships are the same and have one identity. But this is nonsensical as there are two distinct ships – the fixed and the re-constructed. There appears to be no concrete answer as to which is the true ship of Theseus.

Thomas Hobbes’ question responds to Plato’s discussion in the Parmenides . He has a similar theory to the transitivity law “one cannot be either ‘other’ or ‘the same’ to itself or another.” This follows on to the idea that the two ‘ships’ can neither be the same, or other, to themselves. As Plato points out, “But we saw that the same was of a nature distinct from that of the one.” This forms a complex argument about the troubling experience of dual identity.

This topic of discussion begun by Thomas Hobbes has continued centuries later, in the contemporary world. Duality of identity is a problem addressed in the modern television series WandaVision which is explored below.

Shared Identity: WandaVision

You may have heard of the Ship of Theseus thought experiment in the popular television series WandaVision , part of the Marvel cinematic universe. Clearly, Western thought is still supremely puzzled and intrigued by the paradox.

In the TV series, the character named Vision, is a synthezoid: he has a corporeal body with a mind that is created out of artificial intelligence. Like the ‘ship’ in Theseus’ Paradox, Vision loses his original body, but his memories live on in a replica body. The old components of Vision’s old body are reassembled to create a White Vision. Therefore, this White Vision has the original matter, but not the memories. Whereas the Vision has a new body but retains the memories.

In WandaVision , the Ship of Theseus is summarized thus, “The Ship of Theseus is an artifact in a museum. Over time, its planks of wood rot and are replaced with new planks. When no original plank remains is it still the Ship of Theseus?” 

This draws from Plutarch’s version of the thought experiment, calling into question the identity of the ship. Clearly, there have been no decisive solutions to the paradox from antiquity to the modern era. The ambiguity of the ‘answer’ to the Ship of Theseus thought experiment allows modern audiences to continue to interact and respond to ancient philosophy .

Ship Of Theseus: Thomas Hobbes & WandaVision

The television series also includes Thomas Hobbes theory that questions the duality of identity. Vision asks, “Secondly, if those removed planks are restored and reassembled, free of the rot, is that the Ship of Theseus?” This relates to Thomas Hobbes’ idea about reassembling another ship from the discarded parts. The White Vision replies with the paradoxical application of the theory of transitivity: “Neither is the true ship. Both are the true ship.”

Therefore, the two Visions, the one with the memories and a different body, and the other who does not have the memories but has the original body, are both summarised to be one and the same being. But this is impossible because there are two Visions, and they identify differently. Using Plato’s framing, the “nature” of Vision is “distinct from” that of the other one, the White Vision.

The Vision attempts to propose a solution, “Perhaps the rot is the memories. The wear and tear of the voyages. The wood touched by Theseus himself.” This now argues that perhaps neither is the original ship of Theseus, because the original exists only in the memory of Theseus and the people who encountered the very first ship. John Locke’s theory of memory being the creator of identity pieces together the conundrum in WandaVision . The Vision is able to transfer his memories (or ‘data’) to the White Vision, yet the two Visions still identify as separate beings.

WandaVision’s allusion to memory is less of a scientific approach and instead romanticizes the art of thinking. The word philosophy itself means the “love of wisdom,” from philos “love” and sophos “wisdom;” it exercises the thoughts of those who entertain it. The Ship of Theseus thought experiment has certainly exercised many minds from antiquity to now.

Read This Guide Before You Travel to Athens, Greece

By Bethany Williams BA Classics and English, MA Literature Bethany is a Masters student, currently studying the adaptation of Greek myth in modern literature. She is a graduate of Classics and English (BA), during which she studied Ancient Greek language, classical reception within its own time and throughout history, as well as Greek and Roman history. Apart from her studies, she has an appreciation for art, philosophy, and travel. She may be based in England, but her heart is in Greece.

Read more by Bethany Williams

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What is the 'ship of Theseus' thought experiment?

The Greek writer Plutarch proposed this question: If a ship's planks are replaced over time due to wear and tear until none of the original pieces remain, is it still the same ship?

Once upon a time — at least according to the ancient Greek writer Plutarch — the hero Theseus sailed from Athens, Greece, to the island of Crete, where he slayed the half-man, half-bull Minotaur before sailing back to rule Athens. 

The wooden ship that Theseus sailed on, Plutarch imagined, must have become a national treasure, and he posed a thought experiment that has fascinated philosophers ever since: If you repaired Theseus' ship plank by plank so that no original planks remained, is it still the same ship? 

"The people who've been sailing the ship will say, 'Yeah, it's the same ship! We've been sailing for years, and we just keep fixing it,'" said Michael Rea , director of the Center for Philosophy of Religion at the University of Notre Dame. 

"But you can imagine a collector wanting to put the original ship in the museum," Rea told Live Science. "She goes and gathers up all the original planks, rebuilds them and says, 'I've got the ship of Theseus!' So, the question is, which one is the ship?"

Related: What is Occam's razor?

Variations of the ship of Theseus thought experiment pop up everywhere. In Marvel Studios' " WandaVision ," Vision comes face-to-face with a duplicate self and must figure out who is the real Vision. In " The Good Place ," Chidi Anagonye lives hundreds of separate lives and must confront which, if any, represents his real self. In other eras, people have asked if an ax still counts as George Washington's ax if both the handle and the ax head are replaced. 

"It looks like just a dumb party puzzle, right?" Rea said. "But you can learn a lot by thinking through these puzzles really carefully."

The ship of Theseus thought experiment raises questions about the material composition of objects: Is the ship the sum of its planks, the sum of its sailing history or both at once? Can objects made up of other objects even be said to exist? 

To answer those questions, philosophers must deal with conundrums such as whether two things can occupy the same place at the same time, how parts relate to a whole and how to think about the nature of time. 

One answer, Rea said, is to say that only the planks are real and the ship "is just a phase." Taken to its logical extreme, this answer, nihilism, implies that only fundamental particles exist; objects made up of multiple parts are just an illusion. 

But perhaps ships do exist; if so, maybe they are defined by their parts. In that case, Rea said, "the museum curator is correct." If objects can survive part replacement — our cells, for example, are constantly dying and being replaced — then maybe the ship at sea is the real ship.

Or maybe there have been two ships all along, sometimes sharing the same location. In that case, Rea said, "The words 'ship of Theseus' were ambiguous, and that's why now we're all confused."

If two objects can be in the same place at the same time, it opens up a whole new can of worms — space-time worms, to be exact. 

Perhaps the ship of Theseus exists as many overlapping chunks of space-time: The ship in the moment when the first plank was replaced, the ship as it was when Theseus walked the decks and the ship's entire existence, from the growing trees that become its planks to its afterlife as a philosophical thought problem. 

Related: Where does the concept of time travel come from?

All of these ships together can be said to "perdure." "If I perdure, I'm this four-dimensionally extended thing," Rea said. "People started calling those space-time worms." This view usually goes hand-in-hand with the idea that the past and the future exist — a philosophical stance called four-dimensionalism — in contrast to presentism, a theory of time in which only the present moment is real. 

None of this philosophical groundwork, however, can definitively say which ship is the real ship.

— What is the 'call of the void'?

— Why are humans so curious?

— Can we think without using language?

"I think that it is very interesting that we have this question at all," said Anne Sauka , a philosopher at the University of Latvia. The question itself assumes a particular ontology, or theory of being.

She told Live Science that the ship of Theseus puzzle makes the most sense in the context of substance ontology, in which objects are the focus of philosophical interest. The alternative is a process ontology, which sees change as more fundamentally real than objects. 

Seen that way, the planks, the ships and Theseus himself are not static things but rather processes that are always shifting. Trying to claim either ship for Theseus demonstrates an unwillingness to let either ship evolve into something new. "The question itself shows that we have a problem with change," Sauka said. 

The ship of Theseus can also be seen as a metaphor for the self: "If we change, are we a different person?" Sauka said. In process ontology, change is the starting point. "Selfhood is something that only happens by virtue of change," she said. Death "is just the dissolving of that particular process that was a stable process for a time."

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Meg Duff is a freelance science journalist and audio producer based in Brooklyn and the San Francisco Bay Area. She holds an M.F.A from New York University's Arthur L. Carter Journalism Institute. Her stories have also appeared in Slate Magazine, Bedford+Bowery, the climate law podcast Damages, and Apartment Therapy.

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• ogrfnkl "Theseus' ship" is merely a FUNCTIONAL DESCRIPTION of all of its material components (or, rather, the thinker's sensory and/or imaginal representations thereof) as it exists at any point in time in the mind of the thinker. For example, if all of the ship's planks, whether the original ones or their replacements, were to be dismantled and thrown into a pile, the latter could no longer be considered as "Theseus' ship," even though all the components of what the ship used to be are still present, as these components no longer perform the function described by the word "ship." In this sense, the ship is but a consensual mental representation, loosely shared by one or more thinkers, of an assembly of objects that serve a particular purpose. Whether or not that qualifies as an "illusion" would be up for debate. Reply
• Chris Crawford I'd suggest that this is a semantic problem, not a philosophical problem. We're trying to squeeze a set of circumstances into a term into which they just don't fit. Like an overweight person trying to fit into a too-small item of clothing, we twist and turn and squeeze and contort in our efforts. The solution to our dressing problem is to get another item of clothing. Instead of wondering which of the two ships is the "real" ship, we need to acknowledge that there are two entities. One is a functioning ship made out of recent material, and the other is a collection of parts that were once part of a functioning ship. Arguing over which ship is the original ship is an exercise in silliness. Just acknowledge the reality of the situation and get on with life. Reply
• derekamoss Kinda off topic. There is a book called "S". by JJ Abrams and Doug Dorst. It's ARPG actually where you have come across a library book called, Ship of Thesus where two individuals make notes on the book pages trying to figure out who the true author is. The book itself is interesting enough. It also includes codes and puzzles you can decipher. Every time I hear about Ship of Thesus I think of this book. Really a lot of fun. Reply
• indianfoodyummy I think the answer is best provided by 2 measures: 1) If this is Theseus' Ship... at any point that Theseus is present as owner, the description of any variant is correctly described. Functionally, this does allow for multiple ships, although realistically, only one ship of a single identity, prior to his last visit/command of "the" ship 2) whenever the object (i.e. the ship) retains A) a majority of parts comprising the state being measured (i.e. is it a ship) or B) if the original authority of state (i.e. owner/Theseus) documents a set of components comprising the minimum viable set representing an original unit (e.g. the main mast) of the composable object, ...as of the last moment that condition 1 was measured, it is still the object being measured. Whenever applicable criteria (A or B) cease to be fulfilled, the item becomes "remnants of <the object>". Note that you can have multiple sets of remnants, or combinations of new materials and remnants, but only one set will fulfill criteria (A or B) at a time. In the case of humans, consciousness is the minimum viable set. I think the thought experiment is flawed in that it assumes you can replace a majority of a thing, and also not meet the <presence of Theseus> requirement, and still refer to it as the same reference. Reply
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Ship of Theseus: How to Solve the Ancient Paradox

Chris Meyer

I accidentally discovered the mythological Ship of Theseus in 2005, while watching MTV Germany. The new show, which I’m sure you’ve never heard of, was called Pimp My Fahrrad (German for bicycle) and was modelled on the popular Pimp My Ride franchise. People would give their shabby bikes to a swag workshop crew in Hamburg, Germany who would then transform them into two-wheeled cruisers. With phat tyres, shiny accessories and all the rest of it.

What worked well with cars seemed a bit silly with bikes. Bikes don’t have too many parts. In the end, there usually wasn’t much left of the original except for some structural bits and pieces. Perhaps the show should’ve been called Pimp My Frame instead. But as the philosophers at MTV probably intended, Pimp My Fahrrad inspired me to grapple with the Ship of Theseus and the age-old question of what makes us humans who we are.

What Is the Ship of Theseus?

The Ship of Theseus, also known as Theseus’ Paradox, is a thought experiment about identity and identity change. The question is whether an object that had all of its parts replaced, does fundamentally remain the same. Theseus was a Greek mythical king and founder of Athens. His most notable deed was slaying the minotaur. The paradox named after his ship dates back as far as 75 ACE when it was first mentioned by the Greek philosopher Plutarch:

The ship wherein Theseus and the youth of Athens returned 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 place, 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, Theseus

In its adaptation as a popular thought experiment, the key question is often phrases like this: If all the planks of Theseus’ ship are replaced, is it still the king’s ship? There are two possibilities:

• It’s still the ship of Theseus.
• It’s no longer Theseus’ ship.

Both choices require a fair bit of explaining. But the second one begs the question: At exactly what stage in the restoration process did it cease to be his ship?

The Problem of Two Identities

It is said that English philosopher Thomas Hobbes took this mind-bending conundrum one step further by introducing the problem of two identities. Imagine Theseus had another ship and he asked to have all its planks replaced in a dock. The shipbuilder, however, doesn’t want to throw the planks out and uses them to build another ship in a second dock. We now have four possibilities:

• The ship in the first dock is Theseus’ ship.
• The ship in the second dock is the ship of Theseus.
• Both are the mythical king’s ship.
• None of them is.

You don’t have to be a maritime enthusiast to appreciate the real-life application of this thought experiment. Given that our own identity – who we are and how others perceive us – keeps changing, how should we think about ourselves? Before attempting to solve the identity paradox, we may wonder to what extent a naval thought experiment is suited to answer such a profound question.

What Are Thought Experiments and Why?

Thought experiments are cleverly designed mind puzzles. The Trolley Problem Meme is perhaps one of the best-known ones. They illustrate ethical or philosophical problems which may be impossible to test outside of your imagination. Often posed as paradoxes or dilemmas, thought experiments tend to have clear rules and limited options for action to encourage first-principle thinking. Similar to scientific experiments, they establish a quasi-controlled environment in your mind.

The Purpose of Thought Experiments

However, the point of Theseus’ ship isn’t to ask what else the ship is made of, to nitpick if a second ship could even be built out of the first ship’s planks, or to argue that the entire scenario was unrealistic. The point of a thought experiment is to reduce the complexity of reality. By the same token, they can be rendered useless if they’re designed poorly or just silly. The Buttered Cat Paradox comes to mind.

In saying that, the reality is much more complex and many more factors are involved. What these brain teasers can do is force us to focus on the core of an ethical question. The learnings from such simplifications do not necessarily translate directly to reality. But they can inform our actions and decisions.

Popular Variations

Thought experiments can be beneficial beyond any specific philosophical problem. Stories involving moral dilemmas prompt us to reflect on our existence and perhaps even get more comfortable in everyday paradoxical situations. After all, there’s no shortage of those. So it’s not surprising that some variations of the Ship of Theseus thought experiment have emerged in real life and popular culture.

If you get beamed in Star Trek , do you become a copy of yourself and is that copy still you? Given that many of our body’s molecules are replaced every couple of years, who are you, if not the sum of your parts? If first the handle and then the head of my grandfather’s axe is replaced, is it still my granddad’s axe? No matter how you put this question, it seems like you could make a good case for any answer.

Solving the Human Identity Paradox

Even though I had long forgotten about Pimp My Fahrrad , the metaphysical question about our identity remained. Theseus’ ship was a useful thought puzzle to illustrate the seemingly unsolvable paradox of who we are. But none of the options seemed entirely applicable to the human condition. It took more than ten years until I came across a satisfying solution for the paradox posed by Theseus’ ship. In a lecture by psychologist Dr Jordan Peterson. Speaking about facts, stories and values, Peterson concludes:

You’re going from point A to point B and then things fall apart catastrophically and you’re here. […] So you can think: Well, I’m who I was, that would be one kind of identity, I’m the person I thought I was. That blows apart. Then you’re in this terrible place and you think, oh, I’m the sort of person who’s in this terrible place. That’s another form of identify. And then you can think: I’m not the old person or the person who was in the catastrophe, I’m the new person. But the problem is the new person can fall apart, too. But then there’s a third way of thinking. This is a better way of thinking. I’m not this, or this, or this, I’m the process by which [the transformation] occurs. I know something, it’s not quite right. It collapses, it causes trouble (the collapse), but I regroup, I learn, I regenerate, I put myself back together and it happens again and it happens again. But each time it happens, maybe, you’re a little wiser, you’re a little more put together. Jordan B. Peterson, 12 Rules for Life Tour

Identity and the Eternal Now

Who are we in the face of our growth and evolving identity? Peterson conceptualises our identity as a story and suggests that we therefore shouldn’t look at ourselves as fixed states. Rather our identity is the driving force behind the transformation between the states — for the worse or for the better.

I can’t help but notice similarities to the Zen idea of the eternal now . It posits that the past and the future are mere illusions as there is only what happens in the never-ending moment. Paradoxically, this reduces our story to a single state. Reality is experienced only in the present. The past is in your mind only. The future is speculation.

Identity and Personal Responsibility

Peterson’s thoughts are also a nod to personal agency and responsibility. In his reading, identity would be less defined by where who, or what we are at any given moment. But by how we respond to those states. That is the decision we make to transcend our inevitable suffering and manage to grow in spite of it. Again, it would make for a good Zen story in that it’s precisely our insufficiencies that cause transformations through a ceaseless effort to make things better than they are.

What makes this interpretation resonate? It’s no secret that we tend to be harsher on ourselves than on others. After all, we can’t escape the vivid memories of all the worst versions of our past selves. With other people, however, I found that I rather tend to focus on the trajectory they chose for their lives. What I tend to see, respect and admire in people is their story of having been to hell and gotten back out alive. In other words, I see in them the transformative spirit that drives them.

Solving The Ship of Theseus

With that in mind, we should be able to put Theseus’ ship to rest. The thought experiment seems to be less about the rotten planks and more about the act of taking responsibility for replacing them. It’s about the willingness to respond to inadequacies and to let go of insufficiencies, including the ones that gradually built over time. Had the ship not been cared for, the vessel would’ve eventually ceased to exist due to natural decay. Granted, ships don’t have agency, which is why Theseus has to act as a proxy.

Even though in the original story the king has long passed away, we could argue that the wood was replaced in the spirit of preserving his legacy. Arguably, this wouldn’t have happened if Theseus’ deeds hadn’t had such a profound impact on Greek society. The replacement of the rotten planks becomes a symbol of responsible behaviour. The same goes for our shipbuilder. It’s not about where the planks came from. But about his efforts to turn the discarded material into something new and useful. The Ship of Theseus is now his.

Closing Thoughts

I don’t know about you, but I’m quite happy with this solution. It feels comforting that I can remain myself no matter what life will throw at me in the next ten years. And the best thing is, Pimp My Fahrrad can keep its title.

I'm a writer, analyst and teacher with over 15 years of experience and a background in the military, martial arts and failing at things. I enjoy exploring new ideas and making them more accessible.

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