The notion of identity or “sameness” is fundamental to our understanding of the world. The debate about its nature is as old as philosophy itself. The law of identity is the first of the three basic laws of classical logic, on which mathematics and the rest of scientific knowledge rests. It states that “each thing is the same with itself and different from another,” or more formally, that “X is X” for any variable X.

The law of identity may sound trivial, but the philosophical discussion about identity is very lengthy. What follows is my attempt to unpack a single thread of this discussion involving several interesting thought experiments and some unexpected conclusions.

One of the earliest thought experiments relating to identity is called “The Ship of Theseus,” described by Plutarch, the ancient Greek historian, in Life of Theseus (75 CE):

“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 questions whether Theseus’s ship would remain the same ship if it were to be replaced piece by piece until none of the original planks of wood remained. To the Sophists of Athens, the totally retrofitted vessel would still very much be Theseus’s ship, because such an identity is consistent with a Platonistic, idealistic world view rather than a materialistic one. A millenia and a half later, the philosopher Thomas Hobbes introduced an interesting twist to the thought experiment in his book De Corpore (On the Body, 1655):

“Two bodies existing both at once would be one and the same numerical body. For if, for example, that ship of Theseus, concerning the difference whereof made by continued reparation in taking out the old planks and putting in new, the sophisters of Athens were wont to dispute, were, after all the planks were changed, the same numerical ship it was at the beginning; and if some man had kept the old planks as they were taken out, and by putting them afterwards together in the same order, had again made a ship of them, this, without doubt, had also been the same numerical ship with that which was at the beginning; and so there would have been two ships numerically the same, which is absurd.”

Hobbes concedes that the ship remains the same if all its planks were replaced, but asks an interesting question. What if someone were to have gathered all the old planks only to reassemble a ship out of them? Surely, he argues, this other ship must also be the ship of Theseus. This leads to a seemingly absurd situation in which we now have two ships which are both Theseus’s Ship. There are many variations and further elaborations of this thought experiment, which merit an in depth look, but I would now like to turn to the more interesting discussion about a particular type of identity, namely that of personal identity.

The discussion of personal identity address such issues as, “what makes an individual be the same person over time?” The discussion of personal identity is more interesting than the one about general object identity because we seem to hold personal identity to a higher standard. Whilst it is easy to conceive of a change to the identity of, say, a ship or a nation-state (e.g., “is England the same nation after 1066?”), we would rarely accept a case where a person has changed their identity.

There are four common definitions of personal identity:

Who we are is determined by our body; we are the sum of our physical parts. As long as there is a continuation of our body, we remain the same.

Who we are is determined by our brain. Only the brain is core to our identity and everything else is transient.

Who we are is determined by our mind, or our soul. It is not the physical but rather the mental which is core.

Who we are is determined by our memories. It is our memory which provides a sense of continuity.

The American philosopher Sydney Shoemaker describes the following thought experiment in his 1958 book Self Knowledge and Self-Identity:

“Suppose that medical science has developed a technique whereby a surgeon can completely remove a person’s brain from his head, examine or operate on it, and then put it back in his skull (regrafting the nerves, blood-vessels, and so forth) without causing death or permanent injury …. One day a surgeon discovers that an assistant has made a horrible mistake. Two men, a Mr. Brown and a Mr. Robinson, had been operated on for brain tumors, and brain extractions had been performed on both of them. At the end of the operations, however, the assistant inadvertently put Brown’s brain in Robinson’s head, and Robinson’s brain in Brown’s head. One of these men immediately dies, but the other, the one with Robinson’s body and Brown’s brain, eventually regains consciousness. Let us call the latter ‘Brownson’ …. He recognizes Brown’s wife and family (whom Robinson had never met), and is able to describe in detail events in Brown’s life, always describing them as events in his own life. Of Robinson’s past life he evidences no knowledge at all. Over a period of time, he is observed to display all of the personality traits, mannerisms, interests, likes and dislikes, and so on that had previously characterized Brown, and to act and talk in ways completely alien to the old Robinson. What would we say if such a thing happened? There is little question that many of us would be inclined, and rather strongly inclined, to say that while Brownson has Robinson’s body he is actually Brown. But if we did say this we certainly would not be using bodily identity as our criterion of personal identity. To be sure, we are supposing Brownson to have part of Brown’s body, namely his brain. But it would be absurd to suggest that brain identity is our criterion of personal identity.”

Shoemaker’s analysis of the experiment is based around common usage – the way by which most people would describe the unfortunate Mr. Brownson. He concludes that such use, “to say that while Brownson has Robinson’s body he is actually Brown”, contradicts the bodily definition of identity.

The British philosopher Derek Parfit takes the experiment a step further (basing his argument on that of yet another British philosopher, David Wiggins), and giving it the colorful name “People who divide like an amoeba” (Personal Identity, 1971):

“My brain is divided, and each half is housed in a new body. Both resulting people have my character and apparent memories of my life. What happens to me?”

Parfit makes several non-trivial assumptions: brain transplant is possible, and that transplanting the two halves of the brain produces healthy individuals with the same personality and memories. However, the point of the scenario is not to determine the actual feasibility of such an operation but rather the implications it would have if it could be performed. The surgery has 3 possible outcomes:

I do not survive I survive as one of the two people I survive as both people

The third option is an actual possibility because we know of people who have in fact survived with half of their brain destroyed; there is therefore no theoretical reason why both halves of a brain should not produce two conscious individuals. Parfit describes two ways by which we can explain this awkward situation. He then demonstrate why both lead to absurdities and suggests a third alternative.

The first explanation attempts to treat the two resulting individuals as one person:

“What we have called ‘the two resulting people’ are not two people. They are one person. I do survive Wiggins’ operation. Its effect is to give me two bodies and a divided mind.”

Initially the two individuals share the same set of memories. The problem is that as time goes by, and the two individuals live their lives accumulating a distinct set of experiences, it makes less and less sense to speak about them as one person. The second description treats the two individuals as two distinct people:

“I do survive the operation as two people. They can be different people, and yet be me, in just the way in which the Pope’s three crowns are one crown.”

Parfit claims this explanation seems plausible, at least initially, because we can imagine that person with two disconnected hemispheres would experience two disjoint personalities, in which case we would still be inclined to say it is one person we are referring to. The problem with the second explanation, he notes, is that it involves two bodies and thus alters the meaning of “person” far beyond the common usage.

It seems that no matter how hard we try, a coherent explanation of personal identity for the situation described in this experiment eludes us. Parfit claims that this difficulty is the hallmark of the debate about personal identity. Although we’d like to think otherwise, the way we conceive of personal identity is riddled with holes.

In his book Reasons and Persons (1984), Parfit describes a further thought experiment involving a “Teletransporter,” a device similar to the one we know from Star Trek, which illustrates a similar dilemma:

“I enter the Teletransporter. I have been to Mars before, but only by the old method, a spaceship journey taking several weeks. This machine will send me at the speed of light. I merely have to press the green button. Like others, I am nervous. Will it work? I remind myself what I have been told to expect. When I press the button, I shall lose consciousness, and then wake up at what seems a moment later. In fact I shall have been unconscious for about an hour. The Scanner here on Earth will destroy my brain and body, while recording the exact states of all of my cells. It will then transmit this information by radio. Travelling at the speed of light, the message will take three minutes to reach the Replicator on Mars. This will then create, out of new matter, a brain and body exactly like mine. It will be in this body that I shall wake up. Though I believe that this is what will happen, I still hesitate. But then I remember seeing my wife grin when, at breakfast today, I revealed my nervousness. As she reminded me, she has been often teletransported, and there is nothing wrong with her. I press the button. As predicted, I lose and seem at once to regain consciousness, but in a different cubicle. Examining my new body, I find no change at all. Even the cut on my upper lip, from this morning’s shave, is still there. Several years pass, during which I am often Teletransported. I am now back in the cubicle, ready for another trip to Mars. But this time, when I press the green button, I do not lose consciousness. There is a whirring sound, then silence. I leave the cubicle, and say to the attendant: ‘It’s not working. What did I do wrong?’ ‘It’s working’, he replies, handing me a printed card. This reads: ‘The New Scanner records your blueprint without destroying your brain and body. We hope that you will welcome the opportunities which this technical advance offers.’ The attendant tells me that I am one of the first people to use the New Scanner. He adds that, if I stay for an hour, I can use the Intercom to see and talk to myself on Mars. ‘Wait a minute’, I reply, ‘If I’m here I can’t also be on Mars’. Someone politely coughs, a white-coated man who asks to speak to me in private. We go to his office, where he tells me to sit down, and pauses. Then he says: ‘I’m afraid that we’re having problems with the New Scanner. It records your blueprint just as accurately, as you will see when you talk to yourself on Mars. But it seems to be damaging the cardiac systems which it scans. Judging from the results so far, though you will be quite healthy on Mars, here on Earth you must expect cardiac failure within the next few days. The attendant later calls me to the Intercom. On the screen I myself just as I do in the mirror every morning. But there are two differences. On the screen I am not left-right reversed. And, while I stand here speechless, I can see and hear myself, in the studio on Mars starting to speak.”

The careful way by which Parfit decides to unravel events in this thought experiment makes it extremely hard to speak about identity as something equivalent to survival. It also undermines the idea that identity is related to a common set of memories. As long as the teletransporter operates seamlessly, destroying one body while creating another, the notion of identity is preserved (I’m the same person before and after the teletransportation). Once a gap is introduced between creation and destruction, it no longer seems appropriate to speak of survival. Part of the problem is the gap allows the two resulting individuals to develop distinct sets of memories; even if only for an hour.

Parfit argues that the only sensible solution is to entirely give up on the notion of personal identity. We can suggest, he argues, that in both experiments I survive as two people without suggesting that I am these two people. Parfit describes two commonly held beliefs which are at the heart of the debate:

Any question about personal identity, in any describable case, must have a true answer. Important questions rest on the notion of personal identity.

The experiments challenge both beliefs. They directly challenge the first by demonstrating cases where there is no one correct answer to the question of personal identity. The effect is that our notion of personal identity is demoted to that of object identity, for which we’ve already agreed, there sometimes are no clear resolutions.

With regards to the second belief, Parfit claims that yes, there are important questions that do presuppose a question about personal identity. Because these questions are important, the thought experiment does represent a problem. The only way to resolve the difficulty is to drop the presuppositions about personal identity.

Parfit suggests that we change the way we speak about this and similar cases. This is not merely a cosmetic change, but one that has implications on how we behave:

“”Will I survive?” seems, I said, equivalent to “Will there be some person alive who is the same person as me?” If we treat these questions as equivalent, then the least unsatisfactory description of Wiggins’ case is, I think, that I survive with two bodies and a divided mind. Several writers have chosen to say that I am neither of the resulting people. Given our equivalence, this implies that I do not survive, and hence, presumably, that even if Wiggins’ operation is not literally death, I ought, since I will not survive it, to regard it as death. But this seemed absurd.”

The situation is particularly absurd because we would not consider a related case, one in which we “survive” a brain transplant to only one other person, as death. Surly a brain transplant to two individuals is at least as satisfying.

The cases described in both thought experiments are imaginary, but their purpose is to serve as a warning that our notion of personal identity has problematic metaphysical baggage. Although we routinely make consistent use of personal identity in our day to day lives, it does prove problematic in certain real situations, such as that of amnesia or of brain damage.

Parfit’s applies Ockham’s razor to the notion of personal identity. Since it seems to be possible to describe our experiences without presupposing personal identity, the notion must be discarded. People do not exist apart from their components. Parfit describes the change he underwent personally as a result of discarding the notion of personal identity:

“My life seemed like a glass tunnel, through which I was moving faster every year, and at the end of which there was darkness… When I changed my view, the walls of my glass tunnel disappeared. I now live in the open air. There is still a difference between my life and the lives of other people. But the difference is less. Other people are closer. I am less concerned about the rest of my own life, and more concerned about the lives of others.”

If you find the topic of personal identity interesting you should definitely check out a follow up post on the Many Minds interpretation of Quantum Mechanics, and thought experiment. It arrives at a very similar conclusion to the one arrived by Parfit, but from a completely different perspective.

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