According to David Lewis in the “paradox of time,” four thematic concerns arise that seek to explain which of the persons travelling in time is really him. The identity problem refers to the paradox where a time traveler’s personal time coincides with an external time and he meets his younger or older self. These four themes seek to prove the above point and answer the question of whether time travel is practically and theoretically possible and if so, then how can we avoid the paradoxes such as the grandfather’s paradox.
In this regard, how is it possible to identify the real self of this time traveler given that the identity is the paradox of time coincidence between the traveler’s personal time and external time? We must first identify the reason behind the notion how can two events, the traveler’s departure and arrival be separated by two unequal amounts of time. If we successfully address this question, then we are on the brink of identifying which of the persons present are really him. Besides, dealing with the problem of identity problems posed by time travel is much of an answer to identifying the real self, as defined by the time travel concept. Inevitably, it involves a discrepancy between time and time, such that when any traveler leaves and then arrives at their destination, then the time elapsed from departure to arrival is the duration of the journey (Lewis, 1976, p 34). However, if he is a time traveler, the situation is different because the separation in time flanked by departure and arrival does not equal the duration with their order in external time.
Fission can also provide an insight in this case. Consequently, fission can be quite pivotal in tackling the personal identity problems. In the fission experiment, there is the removal of X’s brain from the body of person X. After the brain has been removed, the body of person X is destroyed. In the removed X’s brain, there is a special part called the Corpus callosum. In this region of the brain, there are a bundle of fibers which are involved in the facilitating the transfer of information from one brain hemisphere to the other. Therefore, in fission, this part of the brain is severed. The result of the severing of this part is that two equipollent hemispheres of the brain will be left. With the lower brain, it is divided so that it becomes possible for the transplantation of each hemisphere into one of the two bodies that are qualitatively identical, resulting in Y1 and Y2 as the fission outcomes.
Consequently, according to the alleged intuition, the psychological characteristics Y1 and Y2 are the same as those from X. In this case, both Y1 and Y2 are going to be candidates since they are identical with X. Even if one was absent, the other will also be identical with X. On the other hand, according to the alleged conclusion, fission violates the time travel of identity. This is because the aspect of psychological similarity is considered false. Besides, the question of whether the stage of person X at time t1 and the stage of person Y1 at time t2 have temporal parts of the brain from the same person is in this case dependent on facts that not only concern X and Y1 but also Y2.
As per many suggestions from commentators, fission remains to be an up to standard test to the theories pertaining to the personal identity. The acceptability of the fission scenario however present some challenges for the psychological criterion in particular. This is because both the fission results Y1 and Y2 are psychologically in line with X, and are therefore, both identical, according to the psychological approach with X. Each of the outcomes is however, by congruence, not identical with the other. The Y1 and Y2 have many features in common. However, even at the particular time of the completion of fission operation, they had marked difference especially in terms of spatio-temporal location. Fission disagrees with the transitivity of identity since it shows that there is the possibility of a thing being identical with two other things, which are not identical to each other.
Some commentators claim that because of fission scenarios. It is not sufficient to equate psychological continuity with personal identity. According to these commentators, the non-branching proviso can adequately complement the psychological approach in the sense that even though there would be the survival of X as Y1 or Y2 incase the other was non-existing, the fact that the other exists means that X will cease existing. On the other hand, according to the best candidate clause or the closest-continuer, it is the best candidate between Y1 and Y2 is the one that will bear a lot of resemblance with original X, and hence will be identical with X.
According to Lewis, what matters for survival are the continued existence of your mummified corpse and the continued existence of a spiritual substance? On the other hand, according to Derek Parfit argues that identity is not what matters and that what matters is psychological (Parfit, D. (1971, p 21). He argues that identity is one-one and does not admit of degree and that what matter for survival is psychological continuity. As such, identity is not what matters for survival. In the case of a simple fission and fusion, then psychological continuity is not one-one. He defends his argument that in the case of fission, psychological continuity seems to be a matter of degree. For the case of fission, one item splits up to become two while in fusion, they merge to become one. Parfit believes this is trouble since identity is one-one and “becoming” cannot be identity. The fission case implies that psychological continuity maybe a matter of degree. On the other hand, Lewis uses the case of Methuselah to argue that psychological continuity may be a matter of degree.
The line between fission and identity is clear in Lewis arguments as he states that identity does not always predicate the express properties of maximal fissions of the R-interrelated counterparts. As such, identity borrows from the previous solution of the time travel to predicate express properties of the interrelated fissions, where in time travel, the persistence through time leads to the real self, while in fission, they split to become two. According to Parfit, two people share the same stage prior to fission, something that Lewis disagree and believes that it a matter of psychological continuity. In solving the fission problem, we can apply Lewis time travel solutions. Time travel solutions provided by Lewis depend on external or personal time and real identity to one self. Two people are believed to share the same stage prior to the fission, just like the younger or older self in the time travel. To solve such a problem, one must undergo the systematic steps of time travel for fission. Subject the fission process to two time intervals; one depicting external and the other personal time. The persistence through time and the preserved self-predicate fission is an identity, according to Lewis.
Considering the case he uses to defend his argument on fission, he splits the number of years Methuselah lives to his ability to remember his childhood and progressive memories. At the age of 100, he still remembers his childhood, but new memories fill his old life as he continued to live. At 150, he could hardly remember any memories at his twenties. As the years progress, he can hardly remember his memories and at the age of 969, he can hardly remember any memories of his 850th year. Just like a time traveler, external time changes properties of travel just as it does to existence. Personal time changes nothing and only offers psychological continuity connectedness (Lewis, 1976, p 148).
Assuming that psychological continuity has temporal stages at different times, then the connection flanked by stages of the same person at different times is not identity. This method of solving the time travel to conclude on identity based on external and personal time helps to derive the fission concept. A person must develop through stages in the past through the present to the future measured with both external of personal time. In the future, using external time, so many things have changed and he can hardly recollect his memories because there is no psychological continuity. However, the identity remains his old self via personal time because nothing has shifted to change his real self. Only time is the factor separating the two concepts as Lewis concludes that identity and psychological continuity are compatible and both right, only that the relationships between stages or the same person at dissimilar times is not identity (Lewis, 1976, p 151).
In conclusion, the psychological approach provides clear evidence that the stage of person X, at time t1 are psychologically in line the stage of person Y1 or Y2 at a time t2. This implies that it is the psychological similarity, which is making the time traveler’s personal time to coincide with and external time and thus enabling the traveler to meet his younger or older self. This is also true because the experiment on fission also claim that the only likely difference between the two persons is their spacio-temporal location. Therefore, the fission approach agrees with Lewis conclusion that there is an agreement and compatibility between the personal identity and the psychological continuity in the time traveler.
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