Whether this statement is discussing the identity of persons across time, or the identification of individual persons at a particular time, this statement is wrong from both the subjective and the objective perspectives. To demonstrate this, I will use as my "thought experiment"the "Incident at Nervala IV"(Star Trek: The Next Generation, Episode #150 - "Second Chances") and I'll discuss each of these four arguments in sequence:
1. From the objective perspective, identification at a particular time.
2. From the objective perspective, identity across time.
3. From the subjective perspective, identification at a particular time.
4. From the subjective perspective, identity across time.
Eight years ago, while serving aboard the USS Potemkin, then Lieutenant William T. Riker is beaming back from an Away-Mission on the planet Nervala IV. There is an unexpected malfunction in the transporter system caused by the planet's "distortion field". After a moment's mad adjustment of the transporter control panel, all is well, and Riker successfully beams aboard. Unbeknownst to anyone aboard the Potemkin, however, the transporter signal has been reflected back to the planet's surface.
Eight years later, now Commander William T. Riker is First Officer of the USS Enterprise when it returns to Nervala IV. Who should they discover still on the planet below but a copy of Riker created when the transporter beam reflected back. In order to avoid confusion, the duplicate from Nervala IV chooses to call himself "W. Thomas Riker". He has all of William's memories up to the moment of transport eight years ago. But has had eight years of different experiences since.
Now, according to plot-line specification, the Star Trek transporter can de-materialize an object from one point, and re-materialize it at another. The re-materialization is supposedly accurate down to the quantum level detail. What re-materializes is an exact duplicate of what de-materializes. This is demonstrated by the continuity of memory and personality of individuals being transported. So what does this say about the two Rikers - one of which was transported from the planet's surface to the USS Potemkin, and one of which de-materialized and then rematerialized in place on the planet's surface. Clearly, given the parameters of the "thought experiment", neither has anything "extra" beyond the physical detail that the transporter is equipped to transport. And both were exact duplicates at the time of the "transport" -- down to the quantum level of detail.
Faced with two versions of W.T.Riker, how is Captain Picard to deal with them. It seems obvious to me that one (William T. Riker) is the version that Picard is familiar with as his First Officer. While the other is someone else who is similar to, but not identical with the First Officer of the Enterprise. Suppose that Captain Picard has to reward one of the Rikers for some particularly good deed done in the past. How is he to decide which Riker to reward? Clearly, if the good deed was done before the incident at Nervala IV, then both Rikers deserve the reward. While also clearly, if the good deed was done after the incident at Nervala IV, then only the version that did the good deed deserves the reward. And it is clear which is which. Only one version of Riker could possibly have been at a given spatial-temporal location. Being material objects, both versions could not have occupied the same spatial-temporal location, even in principle.
Now, given the details of the thought experiment (or rather the lack of them), it may not be possible for Picard to physically distinguish between the two versions of Riker. (Although it is certainly possible that in the eight years since the "Incident", one of the Rikers has picked up a scar or two that the other has not.) Let's suppose that there is no physical means of distinguishing between the two versions. Then there remains at least one means of distinguishing between them. The two have accumulated eight years of different memories. Based on that fact, Picard can identify each version based on their apparent continuity of memory. That, and the third-person need to allocate moral responsibility means that the notion of personal identity is neither dispensable nor impossible to determine.
From the instant that two versions of the original become conscious, they will accumulate different memory streams. That fact can be utilized to identify the different versions. And identification of different versions becomes indispensable if one is to assign moral responsibility for their respective actions since the duplication event.
Let's suppose that before his assignment to the USS Potemkin, then Ensign Riker had a lengthy relationship with a girlfriend. Now, many years later, the girlfriend boards the USS Enterprise to be confronted by two versions of Riker. Which is the "real"Riker, and does that question have meaning?
I would argue that it does. And I would argue (contrary to the suggestion of Peter Carruthers in Chaper 7 of "Introducing Persons") that identity across time can indeed be a comparative exercise if the circumstances are sufficiently peculiar. In this case, I would suggest that the girlfriend would identify as the "real"Riker, the version that demonstrates the closest match of "character and disposition of person"(David Hume, An Enquiry Concerning Human Understanding, Section VIII, Part 2) or "psychological continuity"(Peter Carruthers, Introducing Persons, Chapter 7) with her memory of the man she dated. (Mind you, her identification would be relative to her memories. Someone else might consider the other copy as the "real"Riker.)
I also think that our third-person concept of "identity"is not sufficiently robust to withstand some of the challenges of body duplication experiments. Thought experiments of this sort are relatively new to philosophy. And the challenges presented by total body-duplication have not yet had to be resolved in actual practice. So it is not too surprising that our conceptual tool-box is not pre-equipped with the necessary refinements.
In the scenario specified, for example, the girlfriend has at least eight years of separate experiences and memory accumulation to draw upon on which to base her discrimination between the two versions. How would she react if presented with both versions of Riker immediately after the duplication incident at Nervala IV. She would have no basis upon which to choose one from the other as the "real"Riker. She would have to choose arbitrarily (or randomly), but she would still have to choose. Because they are material objects that cannot occupy the same spatial-temporal location, she cannot possibly renew her relationship with both versions simultaneously. (Concurrently would be her closest possible approach to this option.)
This is the most obvious one to deal with. Even if my physical plant could be exactly duplicated, from the instant following the duplication there would exist two individuals with different subjective points-of-view. And those two individuals will proceed to develop two different sets of experiences and streams of memory. Therefore from the subjective perspective, each individual copy will possess a unique point-of-view, and a unique train of apparently continuous memories. Therefore each copy will remain a distinct identity, easily and trivially separable from the others. Each version of Riker will be able to claim incorrigibility about the fact that "He is not I".
Furthermore, if Commander Riker should make plans for a vacation on Raisa, he is making plans for his "I", and not for some version of Riker. So his sense of personal identity is not only incorrigible, it is indispensable to the way in which he thinks about who he is and where he is going.
From this it is clear that from the subjective perspective, the concept of personal identity is obvious, incorrigible, and indispensable. And I would argue further that the way in which we approach the third-person concept of identity is to mimic, as closely as we can, these core features of first-person identity. Hence, our framing of third-person identity in terms of psychological parameters rather than physical parameters.
From the subjective perspective, "I"survive across time if and only if I can trace a continuity of memory back to a previous version of "I", either in one step or in multiple steps. This means that if Commander William T. Riker makes plans to visit Raisa, then it is part and parcel of his plan that it will be his "I"that will visit Raisa. It is not part of his concept of personal identity that it will make no difference if it is his duplicate, Lieutenant W. Thomas Riker, who eventually gets there. The essence of the concept of planning a visit to Raisa, is that once he is there, he will be able to trace a continuity of memory back to the "I"that did the planning. Contrary to the suggestion of Peter Carruthers ("Introducing Persons", Chapter 7), we do not think about the continuity of "I"when planning for the future because there is not currently any issue about the matter. We do not think about it because it is not a possibility, not because it does not make a difference to us. Body-duplication and the potential for future confusion over personal identity is not worthy of concern. Anymore than would be planning alternate travel options in the event that a meteor strike destroys the local airport.
If we think into the past, then "I"-now am the same person as "I"-then if and only if I can trace a continuity of memory from the one to the other. Similarly, when we think into the future, "I"-then will be the same person as "I"-now if and only if the "I"-then will be able to trace a continuity of memory back to "I-now. Incidents involving a loss of that memory continuity create situations where there are two (or more) first-person personal identities involved, even if there is only one third-person identity involved. Multiple personality syndrome is one such case. Amnesia is another case.
The creation of body duplicates will complicate things only when the duplication incident is considered as happening between the plan and the realization. If it was Ensign Riker (before the Nervala IV incident) who made plans to visit Raisa, then does it matter to him whether it will Commander William T. Riker or Lieutenant W. Thomas Riker who eventually gets to Raisa? I suggest that it should not make any difference to Ensign Riker, since both of his future versions will have the necessary continuity of memory to be Ensign Riker in the future. But it will certainly matter the future versions of Riker. For there will be one version who will be able to say, incorrigibly, that "I"did not get to Raisa as planned.
Contrary to the essay question, what thought experiments concerning body-duplication do actually show is that the concept of personal IDENTITY is ultimately indispensable. Arguments to the contrary are based on a separation of the third-person perspective from the first-person perspective. When body-duplication thought experiments are viewed as a completely isolated problem, then one could argue that it would not matter to the Captain of the Potemkin which version of Lieutenant Riker returned to service aboard. And it could be argued to Ensign Riker that it does not matter to him whether it will be Commander Riker or Lieutenant Riker who visits Raisa. So hence perhaps personal identity is dispensable. But such thought experiments need to be considered from both objective and subjective, and both identification-at-a-time and identity-over-time perspectives to get the proper understanding of "Identity". The concept of personal identity subsumes all four of these perspectives on a single consistent reality.
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