In order to answer this question, I will firstly discuss why I believe personal identity is found, or located in, the mind. After establishing this view, I will consider the problem from both Materialist and Dualist positions, focusing primarily on the Materialist perspective in my answer.
Personal Identity
There are three main locations in which it is argued that personal identity is found: i) the soul, ii) the body, and iii) the mind. Many people believe that the soul, if it exists, is non-physical in nature. Unless souls could provide us with a continuity of our existence (for example, through the use of language, or the ability to read, hear, and interact with people), it is hard to imagine how one can identify with it. That is, if souls took a non-human-like form, they could not have a human-like existence. [1] To illustrate this idea, someone who believes in reincarnation, while assured that their soul has passed through countless generations in different bodies and will continue to do so, doesn't identify with someone from the past who had the soul they have now inherited, nor can they anticipate the experiences of someone in the future who will inherit it.
If, on the other hand, our souls did take on a human-like form and had a human-like existence in a non-physical body, then it is possible that there is a continuity of our human existence and thus, our 'sense of self'. However, this would mean that mental continuity is essential for personal identity, and not the soul itself. As can be seen from the example of reincarnation, the survival of a non-physical soul is not sufficient for the survival of persoanl identity.
It is evident that when the soul is defined as the 'essence of the self', what is meant is the soul comprises our mental lives - our thoughts, feeling, memories, and personalities. There is also scientific evidence which shows how mental processes rely heavily on a functioning organic brain [2]. If the brain is damaged or altered in some way, mental processes are influenced, and a person's sense of self, or identity, can be altered.
There is no identity found in the body itself - the body is the medium through which identity can be formed in the mind, but the body is not sufficient for forming an identity. However, the body is essential for forming an identity, since it is the medium through which I understand and perceive the world, myself and those around me. If I was born with a different body to the one I have now, then it is possible that my experience of the world would be different and this would shape and influence my concept of self in my mind, but it is also possible that it would be nearly exactly the same if I lived in exactly the same context as I have with the body that I exist in now [3].
This shows that personal identity is found in mental continuity and not the body because if the mind is fundamentally made up of physical substances, and the physical experiences of my body have an influence over my mental life, then given exactly the same physical experiences, two separate physical bodies should have the same mind. Another way to see how identity is not found in the body is in Perry's dialogue. The characters discuss the idea of a 'body transplant'. Most people tend to agree that the survivor is the brain donor because there is scientific evidence which shows that the mind is intimately connected with the brain [4]. If my brain was moved into another body and my mind is preserved, then I would have mental continuity, and identify with the post-transplant body. The only difference is how I will experience the world, the extent of which depends on how similar, or dissimilar, the post-transplant body is to my original one.
In the Dialogue on Personal Identity and Immortality, Weirob's sense of identity is based largely on her being conscious or aware of herself and her surroundings, which is ultimately dependent on the survival of her brain [5]. This suggests that while the mind and mental processes is dependent on the body (particularly the brain), personal identity is found in the mind and not the body itself. Thus this is a similar case to the one presented above about the human-like soul.
And lastly, another argument for the mind as the seat of personal identity considers how mental separateness doesn't require physical separation - in Dissociative Identity Disorder, (previously known as Multiple Personality Disorder), there is a presence of two or more distinct identities, each with its own unique, and enduring, way of relating to the world and self [6]. While psychological cases such as this one are not definitive, they provide some evidence for the mind as the location of personal identity.
The Materialist View
It is possible that the atom for atom duplicate could be the 'real' person. If the original person's mind and mental states is the product of fundamental physical substances, then it is plausible that they could be reproduced in the duplicate who is composed of the same matter, in the same kind of complex physical arrangements. This leads to the problem of the arbitrary real person - both original and duplicate are physically equivalent, and therefore, mentally equivalent. Each person has sufficient reason to be considered as the 'real' person [7].
The Dualist View
According to Substance Dualism, an atom for atom duplicate is not sufficient as to constitute the original person, for the mind, soul or spirit is a non-physical substance which cannot be recreated through physical substances. Therefore the duplicate may be physically identical to the original yet lacks the original's mind, and could not have the same personal identity as the original.
According to Attribute Dualism, the duplicate could have the same identity as the original, because mental attributes are emergent properties which arise due to the organisation of complex physical matter. However, mental attributes are also irreducible, meaning that unless the duplicate's brain is influenced in the same way as the original's had been, it may not have exactly the same mental attributes [8].
The Original and The Duplicate
For the purposes of the discussion, let the pre-duplication original be called A1; the post-duplication original be called A2; and the post-duplication duplicate be called B. In terms of materialism, both A2 and B will believe they are a particular person, i.e. A1, who has unique memories, thoughts, psychological characteristics, etc. In The Materialist View, I raised the issue of the 'arbitrary real person', but it is not necessarily problematic, as the following argument will show.
Both A2 and B is the 'real person' in the sense that they are both real possibilities of who A1 may become. In the lectures on Philosophy of Mind, the competition problem was addressed and the following choices were considered: either, 1) both persons are you, 2) neither person is you, or 3) one of them is you.
If the second option is choses, it would mean that it is possible to kill someone by duplicating them, which seems illogical [9]. If the third option was chosen, it would imply that there is more to personal identity than the mind itself. But choosing the first option is also not without its problems: After they are created, A2 and B will diverge in their experiences, and therefore memories. A2 will experience and remember different things to B as they continue to exist [10].
Therefore, the duplication of one mind into another body that is physically equivalent (e.g. Gretchen-B), which experiences the world differently to Gretchen-A, makes it possible for the two separate physical bodies to believe they have the same identity yet they do not identify with each other as they continue to exist post-duplication. If this is true, it would show how self-identity does in fact, lie in the mind, as understood by the Materialist.
Personal identity evolves as one experiences life through the medium of a physical body which is constantly receiving sensory inputs. At the time of duplication, there is little differenc between A2 and B - they have practically the same identities, excepting their physically different viewpoints. Their physical, and therefore, experiential differences will lead to the development of unique personalities, impacting on the development of their minds [11]. Both are A1 in the sense that both are real possibilities of A1. A1 will become A2, given A2's circumstances and experiences, and conversely, A1 will become B, given B's circumstances and experiences. Their individual mental development post-duplication means their identities will continue to differ more from one another with time. The idea of 'Person-stages' can be used to understand why A2 believes she is A1 who stayed on the ship, and why B believes she is A1 who was beamed down to the planet [12].
How is A1 both A2 and B?
If the case was that A1 was destroyed and B was the duplicate, then the materialist would immediately agree that A1 anticipated being B, and that B identifies with A1's past.
The only difference in the case where A1 is not destroyed is that more than one possible path for A1 has been created. In such a case, A1 may anticipate the experiences of A2, B or even both.
For example, let A1 anticipate the experiences of B; A1 believes that she will be beamed down onto the planet while her present body is destroyed. B, finding herself on the planet, believes that she was A1 who has now beamed down to the planet. However, if the experiences of B don't occur according to A1's expectations, then A1 anticipates A2's experiences. Another way to understand this concept is to let 'A2' have equivalent meaning to 'Not B'. When the anticipation of B's experiences is not realised for A1 post duplication, A2 identifies with A1's past, which includes the anticipation of experiences that never occured for B.
If on the other hand, A1 had anticipated the experiences of A2, B will still identify with A1's past, which includes the anticipation of experiences that never occured for A2. A2 will also identify with A1's past, which includes anticipating being A2. Therefore, another way of thinking about this situation is that 'B' has equivalent meaning to 'Not A2'.
Conclusion
The answer assumes that two possibilities of the same person can exist in the same time and space, such that, given certain conditions, A1 can become A2 or B. One way to conceptualise this theory is that A2 and B is from two different 'Possible Worlds' [13] of A1's post-duplication future, but in this case they exist in the same time and space dimensions.
In addition, for the post-duplication original and duplicate to both be considered as the 'real person', the question must be considered from a Materialist point of view or similar perspective; it is also necessary for the location of personal identity to be found in the mind/mental characteristics.
Bibliography
Churchland, P. "The Ontological Problem", PHIL 1011 reader, University of Sydney, 2004. [8].
Perry, J. "A Dialogue on Personal Identity and Immortality", PHIL 1011 reader, University of Sydney, 2004. [3], [4], [5], [7], [9], [12].
Robinson, D.J. "A (Partial) Catalogue of Views on Body and Mind", PHIL 1011 reader, University of Sydney, 2004. [11]
Stewart Saunders, 'Reality' lectures, PHIL 1011 semester 2, University of Sydney, 2004. [10], [13]
Websites
A guide to Psychology and its Practice,
http://www.guidetopsychology.com/mpd.htm#3, accessed on 08/09/04. [6]
Baggini, J. "Making Sense of the Self",
http://www.philosophers.co.uk/making_sense.htm, accessed on 10/09/04. [1], [2]