Self Identity Thought Experiments


My last post (seen here) discussed how we traditionally view ourselves and what we traditionally believe constitutes “the self”. We know that people have many different ways of understanding the self, and not all of them see the self in the traditional sense as a fixed entity with unique attributes. If you look up ‘Personal Identity’ in Merriam Webster’s dictionary you will find a vague and almost unhelpful definition. It goes as follows:

The persistent and continuous unity of the individual person normally attested by continuity of memory with present consciousness.”

This definition examines what personal identity is “normally attested by” which makes it quite vague. This alludes to the fact that there are many different ideas about what personal identity is composed of, and that there are many problems with the current theories. To give evidence to the problems and vagueness of personal identity, I would like to present a few famous thought experiments, puzzles, and paradoxes to challenge your perception of personal identity and how you see yourself:

  1. Swamp-man thought experiment11:

This thought experiment written by Donald Davidson goes as follows:

Suppose you go hiking in the swamp and are struck and killed by a lightning bolt. At the same time, nearby in the swamp another lightning bolt spontaneously rearranges a bunch of molecules such that, entirely by coincidence, they take on exactly the same form that your body had at the moment of your untimely death.

The brain and body are structurally identical to your previous brain and body that fell dead just moments before, and the entity that was spontaneously assembled will thus behave exactly as you would have if you did not get struck by lightning in the first place. No one would be able to tell the different.

Question: Would you consider this thing – or person – created in the swamp to be you?

2)  Body “A” and body “B”, written by Bernard Williams12

There are two people – person ‘A’ and person ‘B’ who’s brains are hooked up to a powerful computer and a machine which records the contents of each of their brains and stores it in the computer. The machine also has a brain-programming device which (through some advanced technology) transfers the memories, personality, knowledge and all information content from Person A, to Person B, and vice-versa.

Following the double transfer, the B-body person finds that they now have all of the previous memories of the A-body person. The A-body person also concludes that they have the previous memories of the B-body person.

Scanned by a machine and the informational content of each brain is stored within a computer. Both subjects make remarks about their changed appearance.

Question 1: Did their identity change? Are they the same person as they were before?

Question 2:  What if, after the transfer took place, a surgical transplant  of the brains took place (instead of using the computer and the machine) to reunite the informational content of each of the brains with their original bodies. Would their identities have changed at this point?

3) The Ship of Theseus13:

Theseus is remembered in Greek mythology as the slayer of the Minotaur. For years, the Athenians had been sending sacrifices to be given to the Minotaur, a half-man, half-bull beast who inhabited the labyrinth of Knossos. One year, Theseus braved the labyrinth, and killed the Minotaur.

The ship in which he returned was long preserved. Suppose that, over the next 10 year Theseus continued to use the ship and that it slowly began to wear out and some of the planks needed to be restored or replaced. Each year 10% of the planks were replaced and the old planks were placed in storage, recombined and slowly assembled to form another ship. As time went on more and more of the planks went into storage until after 10 years all of the planks had been replaced and put into storage and assembled to form another ship.

After 10 years it would seem that Theseus has two ships. Firstly Theseus has the original ship which has been reassembled and is sitting in storage. Secondly he has a model ship which was slowly replaced board by board to be an exact model of the original.

Question:  After how many years does the original ship turn into the model ship?


It may seem logical to say that after 50% of the original ship has been replaced then it becomes the model and is no longer the original ship of theseus. Yet this is not founded in any type of logic. What if only 49.9999999% of the ship was replaced? Would an extra square inch of board being replaced suddenly change the ship completely? Also if we define a ship based upon being composed of more then 50% of its original material, what happens when both ships are composed of exactly 50% of the original material? Would Theseus have two ships at this point?

The deeper question is this: if the structure of the original components of an object determine its identify, how much change needs to occur before the object is no longer considered the same object?

4) Teleportation Thought Experiment 

I hope that the previous thought experiments were able to make you think more about identity and what exactly personal identity is composed of. I created the next thought experiment to highlight some of the problems with our current understanding of personal identity over time, and to show the reader that their current understanding of personal identity may contain a few paradoxes. The following section outlines my thought experiment, and the next section discusses the only possible solution.

It is the year 2718, a scientist has just invented a teleporter which looks like the following:

The teleporter has two functions: to dematerialize and materialize objects. It works by first scanning the atoms of an object and storing this information in its hard drive. Once it has the information about an objects location and atomic structure, it dematerializes the object and stores its atoms in the holding tank. It can then materialize the object at any time in any location.

The holding chamber can hold 1 million square meters of atoms, and it sorts them by kind. When the teleporter dematerializes an object and stores the atoms by kind in the holding tank, it does so at random such that atoms are grouped together and its impossible to tell which atoms were just added to the tank and which have been in the tank for a long time. Along the same line, when the teleporter materializes an object and retrieves the atoms from the storage tank, it does so at random such that each individual atom has an equally likely opportunity to be used during materialization.  Dematerializing an object and then materializing it in another location would not guarantee that any of the same atoms are used.

In the picture we see a man being dematerialized and his atoms being stored in the storage chamber. The teleporter has already scanned the man and in less then a millisecond the man will be fully dematerialized and his atoms will be placed in the storage chamber. Once the man is dematerialized we wait a few seconds and materialize him again on the other side of the room.

In this example the same question remains: does the structure of the original components of an item determine its identify, and if it does, at what point does its identify change? In this case, if you believe that the structure of the original components of an object – or person determines its/their identify, then you would believe that the man is dead, and a clone or impostor is in his place. This is one of many different interpretations covered in section 4. To get a better understanding of the root of the problem and the nature of identity, I would like to further this thought experiment by adding a twist.

Teleportation Thought Experiment Addition:

To build on the previous thought experiment, I would like to give the teleporter the ability to dematerialize and materialize parts or sections of objects as small as a single atom. The teleporter has the ability to dematerialize a single atom, store it in the storage tank, and then place a single different atom from the storage tank back in the original atoms place immediately without the object being affected.

The other addition is that you, the reader, are standing next to the teleporter with a scientist who asks you to make a decision. The scientist tells you that the teleporter will dematerialize a percentage of your body, (where atoms are pulled at random from all sections of your body) and then have all of the atoms replaced by atoms from the storage tank immediately afterwards. The scientist asks you to choose what percentage of your body you would like to replace and offers you a reward for choosing a higher percentage  as he is a rich, famous and intelligent scientist. The higher percentage that you replace, the better the reward becomes and it can be anything that you desire. This gives you a great deal of motivation to choose the highest percentage possible so that you know you wont die, and are able to claim the reward.

What percentage do you choose?

This question leaves us with a dilemma as the answer seems to depend on how you define yourself and your existence. In order to avoid mistakingly killing yourself, you need to make sure you have the correct definition of what it means to exist, and what constitutes the self. Once you have confidence in your answer, you can go ahead and give the correct percentage (if one exists).


– Although many people will likely reduce the percentage they feel is correct in order to avoid any chance of death or even choose 0 to avoid the question, the readers goal is to find the highest percentage possible that enables them to continue to exist as the same person.

Click Here to view my solution to the thought experiment.


11. Davidson, Donald. ‘Subjective, Intersubjective, Objective’. Oxford: Clarendon Press, 2001, 441-58 Print.

12. Bernard Williams. ‘The Self and the Future’ The Philosophical Review, Vol. 79, No . 2. (Apr., 1970), pp. 161-180

13. Cohen, Mark. ‘Identity, Persistence, And The Ship Of Theseus’. N.p., 2004. Web. 6 May 2015.