i wrote the following essay on december 17, 2007, for a class entitled "problems in philosophy."

In contemporary philosophy, the problem of identity considers several questions: What makes a person unique? What is essential to a personality and what makes two personalities different? How does a person persist through time? How much change can a person survive? Under what conditions do we have the same person and under what conditions are you the same person from the past?

Examining the question of the persistence of identity over time, we can use the following open-ended statement to help understand the essence of personal identity: Person P at time 1 is identical to Person P* at time 2, if and only if… Contemporary philosophers have finished this statement in various ways. In my paper, I will be examining the spatiotemporal continuity theory, the psychological continuity theory, and how both can lead to the problem of fission/duplication. I will then discuss the three accounts that can be used to examine the problem of fission/duplication: the co-location account, the bi-location account, and the best-candidate account.

First, some important vocabulary must be defined. The problem of identity considers what makes a person numerically similar over time. This must be defined separately from what makes a person qualitatively similar (or different) over time. At age three, I enjoyed listening to classical music. Today, at age twenty-one, I enjoy listening to rock music. I am, therefore, a qualitatively different person. Numerically, however, I am still the same person. To explain this, examine the simple equation: 2+2 = 4. In this equation, 2+2 and 4 stand for the same thing. In the same vein, Jenny at age three and Jenny at age twenty-one stand for the same thing; I am the same being now as I was then, though I have gone through drastic qualitative differences (i.e. my height, weight, and musical preferences have all changed). Like in the 2+2 = 4 equation, Jenny at age 3 = Jenny at age 21.

I will now discuss the two theories used to explain personal identity. The first explanation of personal identity is the spatiotemporal continuity theory. The spatiotemporal continuity theory explains personal identity as a function of spatiotemporal continuity. For example, imagine witnessing a baseball being thrown from pitcher to catcher. You can document the baseball, in terms of space and time, at every moment from when it leaves the pitchers hand to when it reaches the catcher’s glove. This is proof that the baseball that the pitcher threw is the same baseball that the catcher caught. The same goes for personal identity. Theodore Sider summarizes the spatiotemporal continuity theory as follows: “persons are numerically identical if and only if they are spatiotemporally continuous via a series of persons” (Sider 13). That is, if we can follow a person continuously through space and time from time 1 to time 2 (as we did the baseball), that person at time 1 must be numerically identical to that person at time 2.

The philosopher John Locke disagreed with the spatiotemporal continuity theory and used the thought experiment of the prince and the cobbler to demonstrate the opposing psychological continuity theory. The prince and the cobbler thought experiment is as follows. Imagine a prince and a cobbler who each wish to try out life as the other person. One day their entire psychologies get swapped: the psychology of the cobbler is housed within the body of the prince, and the psychology of the prince is housed within the body of the cobbler. According to the spatiotemporal continuity theory, the person in the prince’s body is still the prince and the person in the cobbler’s body is still the cobbler; the theory takes no notice of the psychology within each body. Locke claims that this is implausible, using the following thought experiment.

Imagine that several weeks ago (before the prince/cobbler switch occurred), the prince committed a horrible crime. Now, several weeks later (post-switch), the police go searching for the perpetrator. They determine that it was the prince who committed the crime and arrest the person who appears to be the prince. However, this person whose body is that of the prince, has the entire psychology of the cobbler and has no memory of committing any crime. Meanwhile, the person who appears to be the cobbler (but whose psychology is actually that of the prince and does have a memory of committing the crime) is thrilled to have gotten away punishment-free. Locke states that the only person who should be arrested and punished for a crime is the person who commits the crime (and would, therefore, have memories of committing the crime). Thus, the person with the cobbler’s body should have been arrested even though that is not the body who committed the crime; the psychology and memories of the person inside the body are what matters. Thus, the psychological continuity theory must be used to explain personal identity in place of the spatiotemporal continuity theory.

The psychological continuity theory is formally defined in the following way: “A past person is numerically identical to the future person, if any, who has that past person’s memories, character traits, and so on—whether or not the future and past persons are spatiotemporally continuous with each other” (Sider 15). However, Locke’s theory of psychological continuity was questioned by the philosopher Bernard Williams. Williams used the following thought experiment to lead to the duplication problem. Williams’ thought experiment is as follows.

Suppose a man, Charles, is alive today. Scientists are able to rewire his brain so that his entire psychology is that of Guy Fawkes, a man who died in the year 1606. According to Locke and the psychological continuity theory, Charles is now Guy Fawkes. Now suppose there is another man, Robert, who is also alive today. Scientists rewire his brain in the same way as Charles, and now Robert also has the entire psychology of Guy Fawkes. According to Locke and the psychological continuity theory, Robert is now Guy Fawkes as well. Using a simple transitive property, we see that if Charles = Guy Fawkes and Robert = Guy Fawkes, it must be that Charles = Robert. However, this situation does not make sense; Charles and Robert can not be the same person, as they are two distinct people. Charles and Robert may be qualitatively identical (that is, they have the same memories and characteristics as each other, and as Guy Fawkes) but they are numerically distinct and have been since birth.

This thought experiment, Williams claims, illustrates the duplication problem of the psychological continuity theory: “What happens when psychological continuity is duplicated?” (Sider 16). Because the duplication problem rendered the psychological continuity theory implausible, Williams accepted the spatiotemporal continuity theory as the definition for person identity. However, the duplication problem poses a problem for the spatiotemporal continuity theory as well.

Before explaining the duplication problem in terms of the spatiotemporal continuity theory, we must again examine the theory itself. We said before that the spatiotemporal continuity theory assumes a person is identical to himself between time 1 and time 2 if that person can be followed continuously through space and time between time 1 and time 2. However, suppose this person loses a leg or an arm. This person is now taking up a significantly less amount of space, effectively causing a “spatiotemporal discontinuity” (Sider 16). But we can agree that they are still the same person. Thus, we must define a sufficient spatiotemporal continuity (that is, a minimum amount of spatiotemporal continuity that would be required to assume that a person is numerically identical to himself between time 1 and time 2) in order to allow for cases like this.

The following situation is proposed to define a sufficient spatiotemporal continuity. Imagine you have cancer in the left half of your body, including the left hemisphere of your brain. Doctors can get the cancer out of your body by separating your body into two halves and discarding the entire left half, including the left hemisphere of your brain. The doctors will give you an entire fake left half of your body, and the right hemisphere of your brain will adapt and begin taking over most left-hemisphere functions. All in all, you will be cancer-free and you will, we can agree, be the same person. This must then be our sufficient spatiotemporal continuity: a minimum of half of your body must stay spatiotemporally continuous.

Now let’s look at the duplication problem in terms of the spatiotemporal continuity theory. Going back to the previous cancer scenario, imagine that the doctors actually find cancer in both halves of your body, including both hemispheres of your brain. They can do an amazing operation where your body and brain are split in half (the right hemisphere of your brain stays with the right side of your body and the left hemisphere of your brain stays with the left side of your body). Both sides are operated on to try and rid them of cancer. The operation goes better than expected, and both halves of your body are cleared of cancer. Two people now exist, each with half of your body and one hemisphere of your brain. According to our definition of sufficient spatiotemporal continuity, only half your body and one hemisphere of your brain are needed to deem you identical to your former self. Thus, there now exists two identical people who have come from one person (to whom they are also identical). How can this be possible?

This example demonstrates the same duplication problem as before: how can two distinct people be identical? What, then, can be done to solve this problem of duplication? There are three accounts which attempt to explain and solve the duplication problem: co-location, bi-location, and a third best candidate account.

All three accounts are based on the same premise we discussed earlier: the body and brain of person A are split in half, into persons B and C. The co-location account states that before the operation, there were actually two people located inside of person A. The operation succeeds in separating the two people, B and C. This account seems rather implausible; there is nothing to make it appear as though there are two people located inside of person A. Harold Noonan describes this implausibility as “an undeniable datum of common sense that in this case there is only one person present before the brain transplant, whereas the [co-location] analysis…entails that this is not so” (Noonan). Noonan, however, states that he does not see this “undeniable datum of common sense” as completely undeniable. Though the co-location account may seem to easily solve the duplication problem, its implausibility does not make it a viable option.

The bi-location account of the problem of duplication states that after the operation, person A is bi-located: person A wakes up as both person B and person C. Person A now experiences consciousness from two distinct spatiotemporal locations. The bi-location account is similar to the idea of time-travel, where a person is also experiencing consciousness from two distinct spatiotemporal locations. The major problem with the bi-location account is the implausibility of consciousness having two distinct spatiotemporal locations at a single moment; this account does not seem plausible.

The other major problem with the bi-location account is that it does not seem to truly solve the duplication problem. If A wakes up as both B and C, we can say that A = B and A = C. As we’ve seen previously, this implies that B = C. Thus, we are back at the duplication problem: how can these two numerically distinct people (B and C) be one and the same person? They can not, and we are back where we began. Or, imagine that person B is asleep at some future point in time, and person C is awake at the same future point in time. This would mean that A = asleep and A = awake. As is obvious, this is paradoxical and implausible; one can not be both asleep and awake at the same time. Thus, the bi-location account also does not seem to solve the problem of duplication.

The best-candidate view, then, is to assume that person A goes out of existence when there are multiple candidates for the future identity of A. Thus, person A stops existing after the brain/body split, and persons B and C are now two distinct people. The best-candidate account centers itself around the non-branching view: that “personal identity is nonbranching continuity” (Sider 18). That is, personal identity is stable when a single person at one time is continuous (spatiotemporally or psychologically) with a single person at a past time. In the situation we are examining, this is no longer true; we now have two people at one time who are continuous with a single person from the past. Thus, the two current people (B and C) are no longer identical to the original person (A).

The best-candidate view may seem implausible because of its implication that a double success equals a failure, but this does not cause any true implausibility in the account. Lets go back to the cancer operation discussed previously: you (person A) have cancer in both halves of your body, the two halves are split (B and C), and the doctors operate on each half. If the best-candidate view is to be accepted, you must now hope that only one half of you survives the operation. If only one half survives, you will still be in existence; if both halves survive, you will no longer be in existence (according to the non-branching view). How, then, could two successful operations result in the end of existence of the original person?

While this question may make the best-candidate view seem paradoxical, it is, in fact, the most reasonable account of the duplication problem. The best-candidate view does not seem to contain any implausibility involving common sense or the transitive property (as the co-location and bi-location accounts do). Most importantly, the best-candidate account ensures that we will not encounter the duplication problem; there now exists two people (B and C) who may be qualitatively identical but are surely not numerically identical.

As I have discussed, the duplication problem is problematic for both the spatiotemporal continuity theory and the psychological continuity theory. The duplication problem allows for two numerically distinct people to be proven identical to both each other and a single past person. The co-location, bi-location, and best-candidate accounts attempt to solve the duplication problem. While all three have both costs and benefits, it is the best-candidate account which gives the most plausible solution to the duplication problem with the fewest costs.