The Ship of Theseus and the Question of Identity

Where does consciousness exist, and what is the reality of our soul if we question the basis of personal identity?

| November 2013

  • The Ship of Theseus was rebuilt over the centuries as wood rotted and broke, so at what point did it stop being the original, and when did it become something else?
    Photo By Fotolia/Netfalls
  • With "The Outer Limits of Reason," Noson S. Yanofsky discusses the limitations of computers, physics, logic, and our own sense of personal identity.
    Cover Courtesy The MIT Press

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. If this is the case, where lies our identity and psyche? Should we be held accountable for the actions of another body, or another mind? These questions, and many more, are contemplated in The Outer Limits of Reason (The MIT Press, 2013). Noson S. Yanofsky considers what cannot be predicted, described, or known, and what will never be understood. In this excerpt from “Philosophical Conundrums,” Yanofsky references the Ship of Theseus before questioning the existence of personal identity.

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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?

5/17/2018 4:06:30 PM

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.

5/17/2018 4:06:29 PM

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.

11/7/2015 6:34:05 PM

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.