What is Identity?

The Word Identity

iden·ti·ty (I-'den-t&-tE) comes from the latin "identidem" which itself is a contraction of "idem et idem" and stands for

(taken from Merriam Webster Online Dictionary. Entry on "Identity")

Numerical Identity

Leibnitz coined "numerical identity" stating that A and B are identical if all properties of A and B are equal. 

 A = B iff properties(A) = properties(B)
This kind of identity is also referred to as "strict identity".

Physical Identity

Physical Identity is similiar to numerical identity as both say that two objects are identical if their properties are equal. The difference, however, is that physical identity says that an objects properties are defined by physical attributes whereas numerical identity is defined by numerical value only.

An obvious way to define identity of a diamond would be by looking at it's molecular structure. If I had two stones with exactly the same atomical layout, they would be identical. However, identity is also defined by location in space and time. One object can only be at one place at a time and two or more objects can not be in the same place at the same time.

However, if identity is defined by physical structure and you drop a diamond, it falls on the floor and a small piece breaks away, the definition shows a flaw. While the physical structure has undoubtedly changed (one piece is missing now altogether), the rest is still the same diamond.

We have the same problems with trees or any other kind of animate objects that change their physical structure all the time. While the physical structure of the tree you know to be in the park has surely changed since the last time you've been there (new leaves may have been grown and others were lost), but you would still regard it identical to the tree that stood there the last time.

So, while this definition may be accurate for inanimate objects to a certain degree, it is surely inaccureate for animate objects.

Physical Continuity

We have to introduce the continuity of physical states (Physical Continuity) as identity condition.

An object A is identic to object B if and only if there are physical states between them so that every following state is largely identical to the previous one. 
 identical(A,B) iff A ~= A(1) and A(1) ~= A(2) and ... and A(N) ~= B

For our tree this would mean that throughout its continuous life cycle the tree always stays the same but once it's chopped down and burnt as firewood, no other tree on earth can be that very tree.

Identity of Persons

At first sight, physical continuity seems to be a satisfying identity condition for humans as well: throughout our life cycle we stay the same human as long as there are intermediate physical states that act as links, even if we cut our hair or lose a limb.

If a small blonde girl comes up to me and claims to be my uncle charlie, whom I remember to be tall and of black hair, I would not even consider the possibility that she is telling the truth, because my intuition tells me that it is impossible.

However, if the girl undermines her claim by telling me things only uncle charlie could know (and would never tell anyone, not even a small girl), I would start considering that it could be uncle charlie.

Imagine you wake up one day and go to the bathroom to see that your look has changed. You were put into another body over night. While this is very unlikely to happen, it's strictly not unthinkable as we have just proven. Intuitively you would say that you are still the same person you were yesterday, although there is no physical continuity between the two bodies. So what defines identity then?

John Locke therefore coined the term of mental continuity which defines the identity of persons in addition to physical continuity which defines the identity of everything else. Locke distincts between humans as beings (physical continuity) and humans as persons (mental continuity).

If a prince and a cobbler were body-swapped so that the prince-body now has all the cobbler's memories and the cobbler-body has all the prince's memories, the cobbler would then act as the prince. Locke does not say that the prince in the cobbler's body is the same man he was before, rather he says that there are two kinds of identity: physical identity for human beings and mental (or psychological) identity for persons.

Mental continuity is defined by the memories a person has. If a person B has most of person A's memories, they are identical persons. Mental continuity is also referred to as psychological continuity.

 personal_identiy(A,B) iff memories(B) superset_of memories(A)
"Whatever substance begins to exist, it must, during its existence, necessarily be the same: whatever compositions of substances begin to exist, during the union of those substances the concrete must be the same;"
-- John Locke

(Of Identity and Diversity, John Locke)

A boy gets whipped as child, gets older and becomes a soldier. The soldier is very brave and still remembers being whipped as a boy. Later he gets promoted to general because he was so brave. The old officer remembers being brave but forgot about being whipped as a boy. According to Locke's definition, the boy and the general are two different persons.

This example by Thomas Reid shows that Locke's definition is erroneous, too. To put the brave officer example into a more modern context, Locke's theory fails for people suffering from the Alzheimer's diseasy, or people who lose their memory (or parts of it), etc...

In his work "Reasons & Persons", Derek Parfit presents a solution to this problem. Much like with physical continuity in the physical sense of identity, there is something like mental connectedness or continuity that states a person P1 is the same person as PN if there are N-2 person states between. If every person P(i+1) remembers "many" of the mental states of the person P(i), the persons P1 and PN are identical.

  personal_identity(P(1),P(N)) iff mental_states(P(N)) partly_include mental_states(P(N-1)) and 
                                   mental_states(P(N-1)) partly_include mental_states(P(N-2)) and 
                                   ....
                                   mental_states(P(2)) partly_include mental_states(P(1))

Parfit also states that mental connectedness should not be limited to memory but to all mental states (e.g. taste, sense of humour, knowledge, ...) including memory.
By this definition the general and the boy would be the same person, although the general forgot about the whipping over time.

Objections to this Definition

Williams' Modified Prince/Cobbler

Bernard Williams critizes Locke's whole idea using a modified version of Locke's own prince/cobbler example:
Suppose 2 persons, A and B, are told that their brain (memories, etc...) will be transferred to each other. after that process, one of the the bodies will be tortured, while the other one will receive $100.000,- They both shall decide who will get which treatment.

According Locke, a rational decision seems to be, that A will say that A should be tortured while B should get the money, and vice versa.

Williams then comes up with another experiment:
a person is told that he will be tortured the next day. A plausible reaction of the person would be to feel fear. Then the person is told, that prior to the torture, it's memory will be modified, so it won't remember being told beforehand. The person will still be afraid of the torture.
Then the person is told that his whole memory will be erased. According to Williams, fear is still a reasonable reaction. Even if the person is now told that another person's memory will be transferred to his head, the fear stays and is reasonable.

The experiment is exactly the previous one without mentioning the brain state transfer from person A to person B. So if fear is still a reasonable reaction, person A will this time choose the other option in contradiction to Locke's original experiment, proving that Locke's theory is incomplete, or wrong at all.

However, one might argue that there are flaws in William's arguments. One could say that A doesn't fear the torture but having his mind wiped...
This is what we think, however this depends on your personal point of view.

Parfit's Descendant Selves

Let's have a look at another aspect of the experiments: What if we stopped the experiment in the middle, so that only A is transferred to B and B's memory is wiped for ever. Who would we then say is B? obviously it can't be A, since A is still A. Isn't it possible that B is still B?

According to Derek Parfit, the outcome of a brain state transfer (BST) of A to another body B is not the same A, but a new person called a descendant selve of A. Even in the event of a body-swap as in Locke's original experiment, the outcome would not be the prince and the cobbler with simply their bodies exchanged but new descendant selves of the prince and the cobbler.

Parfit states that the problem has to be separated from the problem of identity altogether.
He further states that "this is not a case of indeterminacy in identity, but a case that we just don’t know how to describe using 'identity'". (taken from "Parfit: Personal Identity", http://arts.anu.edu.au/philosophy/academic/garrett/PHIL2060/LecParfit.html); Parfit suggests a separation of survival and identity. One can survive as two different people without actually being any of them.

Shoemaker's Objections

Sydney Shoemaker critizices Locke's and his supporters' views that identity is based on memory. He states that the logical relationship between memory and identity is the other way round: memory is definable or analyzable in terms of identity.
"The assertion that a person A remembers an event X can plausibly be analyzed as meaning (1) that A now has knowledge of X, (2) that A's knowledge is not grounded inductively or based on the testimony of other persons, and (3) that A witnessed X when it occured. To know with certainty that A remembers X, it might be held, we would have to know all three of these conditions were satisfied, and we could know that (3) is satisfied only if we had a criterion of personal identity by which we could judge that A, the person who now has knowledge of X, is identical with on of the persons who witnessed X. Obviously our criterion of identity here could not be the fact that A remembers X, for we could know this fact only if we had already established that such an identity holds."
-- Sydney Shoemaker, "Personal Identity and Memory"
Shoemaker claims that we are using bodily identity as criterion for identity in the first place to define memory, so memory can not be the sole identity criterion alone.

Our Opinion

I (Viktor Pavlu) think that the two persons A and B are identical only in the very moment after the brain state transfer (BST) if it is possible at all. After that, both are exposed to different environments influencing them to become different identities. Much the same as a single identity changes over time as a result of being exposed to its environment.

I (Gerhard Zlabinger) think that the thoughts and theories concerned with the problem of identity are entirely of hypothetical nature. There is no scientific proof of any kind for the assumptions the theories are based upon, that 'Brain States' are measurable and transferrable. It is my personal belief that this is not the case. However, if it turned out to be true, a great number of ethic and moral problems would arise.