Among all the issues of metaphysics, the question of personal identity is one of the sources of greatest debate. A person's identity is the aspect of their being that makes them who they are and not someone else. Every individual has their own identity, and we recognize that no two people can share one identity and every person must have exactly one identity. Self-identity is a kind of vantage point or subjective center from which we view the world.
In trying to establish personal identity, several criteria must be satisfied. First, we need an account of self-consciousness. This is the awareness people have of who they are, which allows me to talk about myself in the first person. Second, there must be some account for change over time. While we undergo changes during our lives, there must be some underlying essence that remains unaffected since we do not consider ourselves as always changing into a new person. Similarly, there must be some explanation for this persisting entity that gives consistency to an individual. The third requirement is for an account for self-transcendence.
By this we mean the ability people have to think about themselves, or to experience memory. Finally, we must account for the idea of privacy: the fact that I have a privileged perspective on my own self, one that no one else can experience.
John Locke presented an account of personal identity based on a materialist model of the universe. As for discerning individuality, he looked to the beginning of existence for an individual. Every person (or animal, etc.) has a distinct beginning in space and time, shared by no other entity. The fundamental basis for personhood in Locke's theory is consciousness. This gives people the capacity for reflection and reasoning, as well as self-awareness. memory and self-consideration also stem from this fundamental element of personhood.
Locke's approach made a clear distinction between people and other sorts of organisms and things. His use of beginning as a way of distinguishing individuals is effective when the beginning can be ascertained, but in many cases this will not be possible. His method seems too narrow, in that two individuals will remain in separate places at all times, no just at the beginning. His standard of consciousness is somewhat unsatisfactory as well. We don't deny someone personhood when they are incapable of conscious thought, as when asleep or unconscious due to injury. This standard would also seem to imply a kind of personhood of degree, in which those who practiced self-reflection and reasoning the most would be more human than others. Thomas Reid accused Locke of confusing actual personal identity with mere evidence of personal identity.
The ideas of self-identity and the identity of persons in general are related but distinct. The problems encountered in trying to describe personal identity are also found in accounts of self- identity. However, there are some considerations in the study of self-identity that do not apply to personal identity in the general sense. When dealing with self-identity, we have to consider both internal and external factors, since we have additional insight into our selves that we lack when dealing with other people. This leads to the nature vs. nurture question. How much of who we are is a result of our genetic heritage, and how much is due to environmental factors, such as experience and setting? Also, we should consider the perceptions of others, and how they relate to our own self-concept.
While problems can be found for any theory that attempts to explain personal identity, I favor a dualistic model. This is in part due to what I perceive as unresolvable difficulties with the other main categories of theory: materialism and other monistic accounts. The chief problem with materialism is that it cannot provide a satisfactory explanation for immaterial aspects of material beings. no one doubts that the people residing in human bodies have imagination and consciousness, but it seems unlikely that these can be somehow explained as mere chemical reactions and hormonal changes. Why is there consciousness at all? It seems that if a scientist were to come across and study a brain, there would be no evidence for anything more spectacular than what goes on in any complex ecosystem or computer. Yet we find emotions, creative impulses, imagination, and self-consciousness. Some non-material factor must be involved in the process; we can't attribute all this to the bouncing of molecules.
Idealistic models are quite unsatisfactory. Berkeley's idea of material objects depending on minds for existence has little explanatory power or supporting evidence. Although materialism is too mechanistic, the material world is an important component of individual reality. A dualistic account can provide for both the material that composes our physical makeup and the non- physical aspects of Mind that play an important role in our understanding of the physical realm.
Taylor argues that, given the possibilities of dualism and materialism, we ought to seek an account for human nature within materialism. He argues that dualism does not resolve the problems in metaphysical explanations, and has itself more to answer for. Dualism has to account for the interaction between the physical and non-physical components of human nature. (Taylor gives little consideration to theories other than these two, creating a sort of false dichotomy. However, other accounts like epiphenomenalism and parallelism presented such difficulties that they can be abandoned without much regret. Epiphenomenalism relegated the entire mental life to the status of an after-effect, while we tend to think that our mental activity plays an important role in the way we live. And parallelism--the idea that body and mind enjoy a coordinated existence without the benefit of direct interaction--sounds much too convenient and imporbable. Every account will have some difficulties, but dualism and materiaism seem to be the most defensible options.)
I don't think the problems Taylor cites with dualism necessitate the rejection of the theory. Taylor seems to reject dualism in the interests of logical economy. There is difficulty in explaning human nature in terms of the body alone already. Why add a soul to the mix and take on the additional challenge of explaning how the two interact and what the soul can do that the body cannot? However, Occam's razor may not apply to this situation so easily. If materialism were able to give a reasonable account for human nature, then it would seem foolish to postulate an immaterial monad to complicate our understanding fo the machine called homo sapien. But that is not the case. As it stands, and Taylor demonstrates this quite handily, there are innumerable complexities and subtle interactions in the person that seem beyond our comprehension. In trying to locate the point at which a mental idea might precipitate into a physical change, Taylor traces a convoluted and poorly understood chain of events in the body ending in an unremarkable physical response. the ultimate origin of this chain is vague, and several steps along the way are terribly complex and little understood.
Why is Taylor so opposed to assigning some of this mysterious activity to a mysterious, and non-physical, entity? Certainly, this creates a burden on the part of the dualist to explain the interaction between the body and the Mind, or soul, or monad.
Taylor does dualism a disservice in pointing out that no satisfactory locus of interaction has been proposed. He is correct in saying that no one would maintain that at one particular point, such as the right earlobe, the "soul" causes a chemical reaction that leads to other physical changes culminating in brain activity or physical action. However, why would we expect an immaterial entity to work through a pineal gland or some particular lobe of the brain? Shouldnt' it rather serve as a kind of guiding force behind the general workings of the Mind? This may seem like a retreat into the shadowy uncharted regions of the brainworks, but it is not altogether inappropriate. If there is a monad, it will be impervious to the tools of science. Therefore, its function must be in some part of the human person that we cannot explain with axons and ion discharges.
All this does not establish dualism as superior to materialism, however. One of Taylor's most salient points (on p. 30) is to show that bodies without souls can have mental activity. Not a rock, but a mouse or frog, by virtue of its complicated nervous system, can have what appear to be feelings or wishes. As the nervous system gets more complicated, the similarity between human feelings and what the animals exhibit becomes more pronounced. Since the human brain is so much more intricate and complex than even a primate's, it seems possible to account for the higher human mental capacities (self-consciousness, freedom, creativy) with this higher neuron count. This may lead to problems outside the theory, however. Our preconceptions about what a person is may lead to some "requirements of the theory" that materialism does not satisfy, such as an agent to participate in the afterlife. Both theories will have to give some account for origins, since the mind seems too miraculous (or mysterious) to have simply come into being.
It is not unreasonable to say that today's computers already possess a degree of consciousness. In fact, we can see rudimentary forms of consciousness in computers in our everyday world. Cars tell us when they are low on fuel or their lights are on. Vending machines "know" when they lack correct change or a particular selection. And computers can be programmed to diagnose problems in their hardware and even modify their software. How accurate is it to call these features "consciousness"? To answer this question, we have to be clear about what we mean by consciousness.
One fundamental aspect of consciousness is self-awareness. In a simple way, this can be reproduced in mechanical devices. A coffee maker that shuts off when its pot becomes full shows a certain level of self-awareness. However, in conscious beings the self-awareness runs deeper than this. People have a very central feeling of "self" that goes beyond knowing about their body and its environment. We feel that we know who we are, and we can try to understand why we think the way we do. This subjective personality cannot be reproduced in a computer, although a sophisticated computer could appear to show signs of having a "self." The Turing test was designed as a way to identify the arrival of artificial intelligence. In this test, a person communicates with two subjects via a teletype machine. One of teh subjects is a person and the other is a machine "pretending" to be a person. When the person and the computer were indistinguishable, the computer was said to have artificial intelligence. Even if a computer could pass this test, however, this would not show that the computer has consciousness. A sufficiently sophisticated machine may be able to emulate human thought to an arbitrarily complex and original degree. This does not prove that the computer understands _why_ it "thinks" the way it does.
My dualistic understanding of Mind-body interaction precludes a computer having an entire complement of personality. Depending on how much of the person is composed of body, however, computers may be able to simulate consciousness to a very close approximation.