In ancient Greece, there was a legendary king named Theseus who supposedly founded the city of Athens. Since he fought many naval battles, the people of Athens dedicated a memorial in his honor by preserving his ship in the port. This “ship of Theseus” stayed there for hundreds of years. As time went on, some of the wooden planks of Theseus’ ship started rotting away. To keep the ship nice and complete, the rotting planks were replaced with new planks made of the same material. Here is the key question: If you replace one of the planks, is it still the same ship of Theseus? This question about a mythical ship is the poster child for one of the most interesting problems in all of philosophy, namely the problem of identity. What is a physical object? How do things stay the same even after they change? At what point does an object become different? When we talk about a certain object and say that “it changed,” what exactly is “it”?
What happens if you change two of the ship’s planks? Would that make it somehow less of the original ship than after one plank is changed? What if the ship consists of a hundred planks and forty-nine of the planks are changed? How about fifty-one changed planks? What about changing ninety-nine of the hundred planks? Is the single plank at the bottom of the ship enough to maintain the original lofty status of the ship? And what if all of the planks are changed? If the change is gradual, does the ship still maintain its status as the ship of Theseus? How gradual must the change be?
We are not answering these questions simply because there are no objective correct answers. Some maintain that changing one plank changes the ship and makes it no longer the ship of Theseus. Others claim that as long as there is at least one plank from the original, it is still the original. There are also those who maintain that the changed ship is always the same as the original ship because it has the form of the original. None of these different positions are wrong. However, there is no reason to say that any of them are correct either.
Let us continue asking more questions about our beleaguered boat. What happens if we switch the old wooden planks for more modern plastic planks? Then, as we change more and more of the planks, the ship will be made of a different material than the original. What happens if the people who replace the planks make mistakes in putting in the new planks and the ship has a slightly different form? Another question: Does it matter who is making all these changes to the ship—that is, whether one group of workers does it or another? If the ship is to be preserved for hundreds of years, then surely many different people will have to be making the changes. What if we make so many changes to the boat that it can no longer float out to sea? Can we still call it the ship of mighty Theseus if it cannot perform the same function as the original?
Such questions go on indefinitely. I will restrain myself and discuss just one more scenario. Imagine that every time a plank is changed, rather than consigning the old planks to the scrap heap, we store them in a warehouse. After some time, all the old planks are assembled into a ship. This new construction is made to look exactly like the old ship with the planks in their original position. Question: Which ship has the right to call itself the ship of Theseus, the ship with the replaced planks or the ship constructed out of the old planks?
A common answer to some of these questions is that the ship remains the same because the changes are gradual. However, it is not clear why that should make a difference. How gradual must the changes be in order for the original ship to maintain its status? Is there a minimum speed limit for changes? To put the question of what is “gradual” in perspective, consider the case of Washington’s ax. A certain museum wanted to preserve the ax of the founding father of the United States. The ax consists of two parts: a handle and a head. As time went on, the wooden handle would rot and the metal head would rust. When needed, each of these two parts was replaced. As the years passed, the head was changed four times and the handle was replaced three times. Is it still Washington’s ax? Notice that here there is no question of the change being gradual. Every time a change is made, half the parts of the ax are replaced.
Our discussion is not limited to ships and axes. A tree is lush and green in the summer and bare and brown during the winter. Mountains rise and fall. Cars and computers get refurbished. Any physical object changes over time. This is the content of Heraclitus’ famous dictum that you cannot step into the same river twice. For Heraclitus, the river changes at every instant.
Physical objects are not the only things that change. Businesses, institutions, and organizations are also dynamic entities that constantly change and evolve. Barings Bank was in existence from 1762 through 1995. In that time, the owners, workers, and customers all changed. The Brooklyn Dodgers have been around since 1883. Their players, managers, owners, and fans have definitely changed. What remains the same about a baseball team? After heartlessly betraying their city of birth, the Dodgers cannot even claim that they play in the same city as they originally did. In colleges, the students change every four years. Even the professors change over the years. The only real heart and soul of a college are the beloved secretaries. But, alas, even they change. Political parties are also not immune to change. The Democratic Party was founded in the 1790s to support states’ rights over federal rights, the opposite of their current platform. Everything changes!
We are not only talking about change. Rather, we are discussing what it means for an object to be that object. What does it mean for a certain institution to be that institution? When we say that a certain object changes, we mean that it had a certain property beforehand and after the change it does not. In the beginning, the ship of Theseus had planks that Theseus himself touched. At the end, there were planks that he did not touch. That is a change in the properties of the ship. Our fundamental question is: What are the core properties of the ship of Theseus? We have shown that there are no clear answers to this question.
This discussion becomes far more interesting when we stop talking about ancient ships and start talking about human beings. Every person changes over time. We grow from infants to old people. What properties does a three-year-old have in common with their eighty-three-year-old self? These philosophical questions are called the problems of personal identity. What are the properties that make up a particular human being? We are not the same person we were several years ago. Nevertheless, we are still considered the same person.
Philosophers usually fall into one of several camps on this question. Some thinkers push the notion that a person is essentially their body. We each have different bodies and can say that every person is identified with their body. By postulating that a human being is their body, we are subject to the same insoluble questions that we faced with the ship of Theseus and other physical objects. Our bodies are in constant flux. Old cells die and new cells are constantly being born. In fact, most of the cells in our body are replaced every seven years. This leads to hundreds of questions that philosophers have posed over the centuries. Why should a person stay in jail after seven years? After all, “he” did not perform the crime. It was someone else. Should a person own anything after seven years? The old person bought it. In what sense is a person the same after having a limb amputated? Science fiction writers are adept at discussing challenging questions like cloning, mind transfers, identical twins, conjoined twins, and other interesting topics related to the notion that a person is the same as their body. When an ameba splits, which is the original and which is the daughter? When your body loses cells it loses atoms. These atoms can go on to belong to others. Similarly, other peoples’ atoms can become part of your body. What about death? We usually think in terms of the end of a person’s existence when they are dead even though the body is still there. Sometimes we use sentences like “She is buried there” as if “she” were still a person. And sometimes we use sentences like “His body is buried there” as if there is a difference between “him” and his body. In short, it is problematic to say that a human being is identified with their body.
Other thinkers favor the notion that a person is really their mental state or psyche. After all, human beings are not simply their bodies. A person is more than a physical object because there is thought. To such philosophers, a person is a continuous stream of consciousness—they are memories, intentions, thoughts, and desires. This leads us to ask other insoluble questions: What if a person has amnesia? Are they the same person? Doesn’t a person’s personality change over time? Who is the real you: the one who is madly in love with someone or the one who is bored with the same person two months later? Literally hundreds of questions can be posed about change in a person’s thoughts, memories, and desires. Again, philosophers and science fiction writers have become quite adept at describing interesting scenarios that challenge our notion of a human being as a continuous stream of mental states. These scenarios are concerned with Alzheimer’s disease, amnesia, personality changes, split-brain experiments, multiple personality disorders, computers as minds, and so on. There are also many questions along the lines of the mind-body problem. How much is the mind—that characterizes a human being— independent of the brain, which is a part of the body?
One of the more interesting challenges to the position that continuity of mental states characterizes a human being is the question of transitivity of identity. My mental states are essentially the same as they were ten years ago. That means I am the same person I was ten years ago. Furthermore, ten years ago, my mental states were essentially the same as they were ten years earlier. Hence the person I was ten years ago is the same as the person I was twenty years ago. However, at present, I do not have similar mental states to those I had twenty years ago. So how can it be that I am the same person I was ten years ago, and that person is the same as I was twenty years ago, but I am not the same as I was twenty years ago?
Yet another option is that everyone has a unique soul that determines who they are. Avoiding the questions of the definition or existence of a soul, let us concentrate instead on how this answers our question of the essential nature of a human being. Assuming the existence of a soul, what is the relationship between the soul and the body? What is the relationship between a soul and a person’s actions, psyche, and personality? If there is no connection, then in what sense is one soul different from another soul?
How can you differentiate between souls—or identities, for that matter—if they have no influence over any part of you? What would the purpose of an identity be? If, on the other hand, a connection exists, then does the identity change when the body, actions, psyche, or personality changes? Is the soul in flux? If the identity does change, we are back to the same questions we had previously asked: Who is the real you? Are you the one with the soul prior to the change or are you the one with the changed soul?
Most people probably have an opinion representing some hybrid version of all three ideologies: a person is a composite of body, mind, and soul. Nevertheless, all schools of thought are somewhat problematic.
Rather than answering all the questions posed in this section, let us try to resolve the issues by meditating on why none of the questions have clearcut answers. Why is it that when we pose these questions to different people, we get so many different answers?
Examine the way people learn to recognize different objects, make definitions, and create distinctions. In the beginning, babies are bombarded with many different sensations and stimuli. As toddlers grow, they learn to recognize objects in the world. For example, when they see a shiny silver thing covered with brown gooey stuff coming toward them, they have to learn that it is applesauce on a spoon and that they should open their mouth. By learning to recognize that the physical stimulus of silver covered with brown gooey stuff is applesauce, they are able to handle life better. Human beings need to classify objects. We learn how to tell things apart and determine when they are the same. We learn that an object still exists even when it is out of sight (“object permanence”). Children learn after a while to recognize their mother. A few months later, they learn that even though she is wearing makeup—that is, even when she looks different— she is still the same person. Children have to learn that their mother is the same even when she is wearing perfume and smells totally different. Here toddlers are acting as philosophers and learning how to deal with different questions of personal identity. With all these skills, children are imposing order and structure on the complicated world they have entered. Before these skills are mastered, they are showered with an incomprehensible stream of stimuli and sensations. With these classification abilities the children can comprehend and start to control their environment. If they fail to learn the classification skills, they will be overburdened with external stimuli and unable to deal with their surroundings.
With enough sophistication, children also learn to classify abstract entities. For example, they might learn what it means to be a family. Their mother is a family member. Their father and siblings are also part of the family. What about first cousins? Second cousins? These are a little vague. Sometimes they are part of the family, and sometimes not. Children must learn what is a family and what is not. As they grow, they learn to classify even more abstract entities like numbers and political parties.
Not only do children learn to classify objects and people, they also learn to name them. They realize that they live in a society of other classifiers, and in order to communicate with these compulsive classifiers, they follow their example of giving names to objects. They first give the external stimuli their own names. As their communication skills progress, they learn to forgo their names and start to use other people’s nomenclature for objects. They call brown gooey stuff “applesauce.” They learn to call the woman who takes care of them “mom,” regardless of her wearing makeup or not. By using the same names as others, children are showing society that they are conforming to the prevailing classification system and that their mental processes are similar to those of others. Society then rewards them by showering them with love and providing the protection they need.
The point is that classifying and naming are learned skills. Children do not learn exact definitions of things because they are never exposed to exact definitions. They learn to classify and name physical stimuli. Some notions are exact and unchanging. The concept of the number 4 is exact and has a clear definition. In contrast, many other notions lack sharp definitions. The first part of this section shows that even physical objects do not have sharp definitions.
With this in mind, we can discuss the many questions posed at the beginning of the section. Is the ship of Theseus the same after changing one plank? The proper response is that the definition we have for the ship is not clear enough to provide an answer to that question. There is no exact definition of the ship of Theseus. We only have what we learned—that is, the stimuli we were taught to associate with the ship.
The ship of Theseus does not really exist as the ship of Theseus. There is no exact definition of what is meant by the ship of Theseus. It exists as a collection of sensations but not as an object. Yes, if you kick it you will feel pain in your toes. When you look at it, you will see brown wood. If you lick it, you will taste stale wood and salt water. But these are all just sensations that one learns to associate with something we call the “ship of Theseus.” Human beings combine these sensations and form the ship of Theseus. Of course, the ship exists as atoms. But it is made of atoms as atoms. The atoms are not tagged as the ship’s atoms. Rather, it is we who make those atoms into a whole entity called a ship. It is we who further demarcate this ship as somehow belonging to the mythical general Theseus. The first part of this section cited many examples demonstrating how the ship can lose and change atoms and still be the same ship. It’s all in our mind. We are fortunate to live among other people who learned to give the same names to commonly occurring external stimuli. Each of us calls these similar stimuli the “ship of Theseus.” Since we all agree with this naming convention, we do not commit each other to insane asylums. Nevertheless, the existence of the ship of Theseus is an illusion.
Reprinted with permission from The Outer Limits of Reason: What Science, Mathematics, and Logic Cannot Tell Us by Noson S. Yanofsky and published by The MIT Press, 2013.
Originally Published: November 2013
The Ship of Theseus Paradox is fallacious. Although it is treated as a serious metaphysical question by mature, learned people. “If you replace a part of the ship of Theseus, such a plank in the hull, is it still the Ship of Theseus?” The question can’t be answered until the questioner first answers the question: “What defines the Ship of Theseus?” That should have been the question. To put it another way, when the ship-builder said to Theseus, “I’ve completed the construction of your ship and here’s my bill,” how did Theseus know that the right ship was built?” The asking of the question pre-supposes that there is some intelligent entity out there, somewhere in the universe that assigns identities to things. There is not. The “Identity” of something is assigned by humans. That human is someone who either “owns” the “thing” or is given authority to assign the identity. What happens is this: The authorized human selects characteristics of the thing that are used as the basis for assigning it an identity and giving it a name. Let me give you an example from the aerospace industry where I have some experience. Let’s say that the Acme Aerospace Company designs, builds and tests a rocket engine that works great. We will call it the LR-123. The Air Force wants to buy some for use in a missile. When each rocket engine is delivered, how does the Air Force know it is getting what it paid for? The system created to ensure that is called “Configuration Management.” The identity of the LR-123 is defined by a “drawing tree” and a list of applicable specifications. The drawing tree lists all drawings that define the components and their assembly. The drawings define the dimensions, tolerances, materials, and arrangement. As long as the engine is built in accordance with all the drawings and specifications, it is an LR-123 engine. Now let’s create a “paradox.” Let’s say we have assembled an LR-123 rocket engine in accordance with drawings. In the process of packaging it, a technician accidentally scratches a valve. So, we replace the valve with another one with the same part number. Question: Is it still the LR-123 rocket engine? Yes. The engine satisfies the criteria for its identity. Let’s complicate that. Before the Air Force will accept delivery of each LR-123 engine, it must undergo a test-firing. So, we test fire an engine. It meets acceptance criteria, but in the process of packaging for delivery, a valve is scratched. It is replaced with a valve with an identical part number. Is it still the same engine? No, the replacement valve was not acceptance tested. The basis for identification had changed. The engine has to be retested. Here is a trivial example. A man leaves his house every morning for work. He walks to a nearby intersection where he boards a bus labeled “Route 161.” The bus takes him through town to the building where he works. One morning at the bus stop he notices that when the bus arrives it has a new set of tires. Is it still the “Route 161 bus? Yes. As long as the illuminated panel in the front says, “Route 161” it is that bus. Getting back to the ship of Theseus. To answer the paradox question, Theseus would have to tell us what were the configuration requirements for his ship before he accepted delivery. Did the wood of the planks have to be of a particular variety? If yes, then any changes in planking later on would have to be that wood or it would not be the ship of Theseus. The same question would have to be asked of every other part of the ship. Another example of how identity if assigned by humans. You take a trip to Italy and visit the Colosseum in Rome. You discover when you see it that It has fallen apart over the centuries. Is it still the Colosseum?
As part of a physical world, we try to reason and address the ourselves with respect to a frame of reference, the human one. The physical world evolves. The evolutionary process is strongly related to the 'Scale of Operation'. The smaller we go, the smaller is the timeframe for evolution. The microbes have even a life span of microseconds. Where as at the other end of large scale structures, the Galaxies have time of change in Lightyears. So the the entire universe is a state of flux, only the reference points for evaluation changes. The time is a tool for convenience and provides measures for evaluation. The human shape is just a combination of mind and body. Though as per Buddha the flux is continuous providing a changing physical entity, the mind body system due to referencing has certain components which slowly change compared physical change. The memory, the experience, the thoughts do change in a reference frame other than physical. So in the realm of mind and body we still remain an entity diffrent than the rest.
This is more straightforward than it seems. The Ship of Theseus remains the Ship of Theseus regardless of changes in planks or rigging just as I am who I am despite having none of the cells I was born with and none of the character as that has been changed by education and experience. We are who we are because we are recognized by others to be the sum of the histories we leave in our wakes.
It makes me think of bands and how the name goes on even without the original players (none of them in some instances).
The ship in question has the same frame regardless of the plank changing; much like a human has the same skeleton. Cell regeneration has nothing to do with a being changing what and who it is. If a human were to make one change/improvement each and every day of it's life until it no longer acted or appeared the same, the human would of but found a more perfect mode of efficiency. Much like the ship, perhaps eventually the boards would technologically be replaced as to not have to be replaced; reducing maintenance, and preparing it for Eternity. As a Human must Prepare for Heaven.
The ship in question would have the same frame regardless of the plank replacing. Much like a human who has the same skeleton underneath the tissue. It is the same but improved. If you yourself improve each day, changing one flaw toward the better; then eventually you can achieve perfection. You have not changed your identity, but your original flawed self. Maintenance(and a more perfect way of operation) is all this conundrum is about.
There are many facets to the subject of identity, but I will limit this response to the question mentioned on the web page: is the ship of Theseus the same after a replacement of a rotting plank with a replacement plank, or after other changes? I submit that there is a straightforward, easily understood answer to that question. The reader does not have to accept this answer because it is written here; I only ask that you follow the line of reasoning and discover for yourself the process of establishing identity described as follows. To begin, this is the question that should have been raised before asking if the ship is the same: what constitutes “sameness”? Consider the following. The universe is not a collection of objects; it is one giant process composed of billions of sub-processes involving the interaction of energy and matter. Even things we refer to as solid are, in fact, processes. Bodies such as planets and stars that appear substantial are composed of particles interacting with other bodies and forces. Since all forms are undergoing this constant state of change every millisecond there can be no identity that is inherent or permanent in any of the forms. There is no universal definition for identity that is valid over any period of time. “Identity” has no meaning apart from the intervention of human thought. The description of an identity requires an observer. Those processes we call “things” have attributes that might also be called features or characteristics that can be sensed or measured by the observer. The observer selects those attributes that serve his purpose. The attributes become the basis for the identity. The observer assigns the identity. After a period of time when changes to the object occur, the observer who assigned the original identity has to decide To illustrate, here are some imaginary scenarios: The coliseum in Rome was built almost two thousand years ago. It is now in a state of ruin caused by earthquakes and stone robbers; large portions of the walls and interior have fallen off, but the citizens of Rome and tourists to this site do not question that this is still the Coliseum. The fact that hundreds of tons of stones and cement are missing is irrelevant. The modern day Romans have assigned an identity. Another imaginary scenario: Vincent van Gogh creates an oil painting that incorporates his unique style. It is sold to an art gallery where it hangs for a hundred years. A van Gogh collector finds it; the paint exhibits oxidation; the colors are not as bright as the day van Gogh completed it. But it is an original van Gogh so he expresses an interest in buying it. The gallery owner shows the potential buyer the papers of authenticity for the painting, but the buyer reads a note that states that during transfer to the gallery an area on the painting was scratched. A gallery employee was able to obtain tubes of the same paint used by van Gogh. The employee touched up the scratched area. The defect was hidden; the area repaired is not visible and not detectable even by scientific means. However the buyer withdrew his offer because it is not “an un-retouched, original van Gogh”. No amount of philosophical argument could alter the reality that it might be the same. It did not satisfy the identity that gave it value. The buyer assigned that identity. A modern day example is found in the aerospace industry where I have been employed. There is a discipline known as “Configuration Management”. It works this way: An aerospace company designs and builds a rocket engine that is demonstrated to NASA. NASA says “I want to buy that engine.” One is built and delivered to NASA, but is it the same engine that they saw test fired? The answer is “yes.” The configuration of the engine (its identity) is defined by a series of drawings. There is a top assembly drawing that references a hierarchy or “tree” of many other drawings. These drawings specify the arrangement of components, choice of materials, dimensions, tolerances, surface finishes, and additional information required to machine or assemble the engine. Anything built to those drawings represents the engine. The NASA representative could go out on the factory floor and pick out an engine, one labeled Serial Number 2001. In the process of getting the engine ready for shipment, a valve is scratched. It is only cosmetic damage, but the company decides to change the valve. Is the engine still S/N 2001? Yes, because it satisfies the drawings, the identity. The replacement of any part does not affect the identity. To further show how the observer assigns identity, NASA now requests that the engine be subjected to an acceptance test firing before delivery. This is done. In the process of removing the engine from the test stand, the same valve is struck by a mechanic. Because the blow may have affected the valve operation, the valve is replaced with an identical part. Is this the same engine? No, even though it satisfies the drawings, its identity, assigned by NASA, requires that every part undergo the test. More examples: as stated on the website “Within the span of seven years, every cell of your body will die and be replaced—you literally are not the same person you once were.” A biologist, using his standards, could rightly agree. On the other hand, a member of a police laboratory checking the person’s fingerprints or, better yet, a DNA sample would state, unequivocally, this is the same person. To raise the philosophical question, “Is this really the same person?” serves no purpose. In other words, the factors of any particular identity are arbitrary and subject to human choice. A single, absolute definition of sameness does not exist in a spiritual realm somewhere in the universe that we can uncover by philosophical reasoning. Now relate this explanation to the original question: is the ship of Theseus in the port of Athens the same after the replacement of the rotting planks? The truth is that the ship of Theseus was never the same from the moment it left the shipyards where it was built. There were continuous changes to the sails, the wood, and the metal parts that comprised the ship. Not all of these changes were visible to Theseus. Change is said to occur when those attributes that can be recognized by the observer are altered or disappear. Whether the ship of Theseus is the same is up to the citizens of Athens.
This article provides a pleasantly readable narrative-driven account of the Ship of Theseus problem, but there are two important shortcomings: First of all, the 7-year whole body cell replacement story is a myth: http://www.livescience.com/33179-does-human-body-replace-cells-seven-years.html Second of all, this articles gives no support for its claims that: "there are no objective correct answers." Perhaps the author is inferring this from the fact that there is disagreement about the correct answer? But it does not follow from the fact that people disagree about a thing that there is no correct answer. The Ship of Theseus is a valuable puzzle that cuts to the heart of important questions concerning the nature of identity; we cannot not infer from the fact that it has been around for a long time "that there are no objectively correct answers..." in fact, to do so would be intellectually irresponsible.