Alan Watts once said, "Trying to define yourself is like trying to bite your own teeth." The task of personal identity is to define a quality of a human which makes him or her unique self. The person whose identity is in question must realize themselves, and other people must identify this person. In other words, what makes John unique from Bob?
One must consider both internal (mind) and external (body) perspectives. There are several general philosophical theories of this identity problem. In the following paragraphs one will find the body theory, soul theory, and a more detailed explanation of the conscious theory. One theory of personal identity is known as the body theory. This is defined as a person X has a personal identity if and only if they have the same body Y. However there are two problems with this definition.
The first is qualitative. It is necessary to have the same body, but if that body is changed, is one the same person? Someone's body is surely different at age 40 than at age 4. Also a problem arrives in alterations to a body. If John goes to war, becomes injured by a mine and then has his legs amputated is he not still the same person, John? Therefore, the preceding definition of body theory is not sufficient, since it does not account alterations to the same body. Yet another problem is numerical.
If someone were to get a finger chopped off, would that finger be considered another person? What if a scientist was to use someone's DNA and replicate another person with the same body? Surely just because there are two identical bodies, these bodies cannot be the same person. They would live two different lives. Therefore, the body theory alone cannot be considered a necessary and sufficient definition when defining personal identity.
Another common theory of personal identity is the soul theory. This theory is: a person is has personal identity if and only if they have the same soul. The problem with this theory is arrived from the definition of a soul. Soul is a very difficult term. It is thought by many to be a spirit that passes from your body into another realm (i.e., heaven, hell, etc.). However, since no one has ever seen, felt, touched, smelled or tasted a soul, it is a mysterious phenomenon. Since we have no clear and distinct idea of a soul, it would not be wise to base the definition upon it. For instance, some religions believe in reincarnation after death. This is when a soul enters another body.
With this in mind, someone's soul such as Elvis could become reincarnated in someone else named John. However, we would not say that this Elvis and John is the same person. Therefore, the definition of the soul theory fails in that this does not become sufficient for defining personal identity.
The most recognized true philosophical theory of personal identity is the consciousness theory. The consciousness theory is believed by most to be the best definition of personal identity according to most philosophers. However, there are three different versions of the consciousness theory that will be discussed in further detail. The conscious self, experiential content, and connected stream of consciousness theories. First, we have the theory of a conscious self…a person has a personal identity if he or she has the same conscious self. In other words, if two people have a different conscious, then they each have personal identity. At a first glance, this would be a good definition of a personal identity. It is analogous to Descartes' cogito, "I think therefore I am." Being conscious would mean knowing that one exists, and able to think about any experience that happens. However the main fault with this is that it is a circular definition. We are using the definiens in the definiendum which is not a good tactic of defining personal identity. Next, a common derivative of the consciousness theory is the consciousness of experiences theory. This is defined as having the same experiential content. This theory is based upon Locke's theory of the mind being blank, and building from experiences. One would have personal identity because only one being can go through the same experiences in a finite space. This theory does solve some problems arrived at by other theories. Say for instance there are two twins.
This would be difficult to explain in the body theory of personal identity, but with the conscious experience theory it would be simple. Although they have identical bodies, since the day they are born they go through different experiences. Therefore the twins do have their own identity since they have both had different experiences. However, there are some problems that arrive with this theory. First, the definiens is not necessary. Say for instance a person has two different conscious experiences. Some people have a multiple personality disorder, but that does not mean they are truly different people, although they may have several identities. Another problem is the definition is not sufficient.
Say for instance a brain was replicated and was put into a computer. At the exact time the brain was replicated, both brains had the same experiential content. Yet the most devastating deviation from this theory is the simplest. The biggest problem is that we cannot consciously remember every experience of our lifetime. For instance, when one tries to remember what they got for their fifth birthday the day after their birthday they would be able to tell another. However, trying to remember what one received for their fifth birthday when they are thirty is most of the time impossible. Also, just because a person may be drunk and does some action does not make him or her another person!
In other words, the conscious experience theory fails because the human mind cannot remember every conscious experience. Last of all, we have the connected stream of consciousness. This theory states that a person's identity is made up of a stream of connected conscious experiences. This theory solves the problem of having a different memory of at different times of life. For instance, when we are forty we certainly have a different memory than when we were four years old. With the connected stream theory though, we are still the same person whether we are four or forty. In general, we have a finite mind, so are conscious is connected in a chronological pattern. To make it simple, it is analogous to a river. If you were to see a river every day, one is not looking at the same particles of water (representing body or experiences), however one cannot dispute that it is not the same river.
Therefore the river represents personal identity. This theory solves most of the problems arrived at from the other two forms of conscious theories. It would seem that the connected stream of consciousness is a definition which is both necessary and sufficient in defining personal identity.
In conclusion, we have discussed the three common theories of personal identity…the body theory, the soul theory and the conscious theory. The main problem with the body theory is that people's bodies change, yet they still have the same identity. The main problem of the soul theory is that it is immaterial, and the whole idea of souls is disputable. Of the three consciousness theories, the conscious self-theory fails in that it is a circular definition. The conscious experience theory fails in that a human cannot be conscious of everything of his or her life. Finally we discussed the connected stream of consciousness theory which is the best description of a personal identity. Personal identity therefore is made up of a connected stream of consciousness (i.e., thoughts, memories, actions) and therefore is always changing slightly. As James Baldwin, a U.S. author once quoted, "An identity would seem to be arrived at by the way in which the person faces and uses his experience."