Scott Daniel
Philosophy – Period 3 – Mr. Goldberg
On Personal Identity and “Nightly Duplication”
The British philosopher Derek Parfit holds that the idea of personal identity, the
persistent “self” with which a human being identifies throughout his or her life, is not as
important as it seems. Rather, Parfit maintains that the criterion of psychological continuity –
smooth progression of thoughts, memories, and character traits from moment to moment –
sufficiently encapsulates every important feature of personal identity. In other words, personal
identity is not a further fact beyond psychological continuity; if I am continuous with my
sixteen-year-old self (if I act today more or less like I did when I was sixteen and I remember
events that occurred when I was sixteen), it is not meaningful to further inquire whether my
sixteen-year-old self was numerically “identical” to the present-day “me.” I wish to illustrate and
further explore Parfit’s critique by means of a thought experiment involving physical death, the
phenomenon most often supposed to be the end of all personal identity.
Suppose that Alfred believes that personal identity is defined by spatiotemporal
continuity (i.e., the persistence of a single physical human being through time). Then if I were to
ask Alfred today whether he is numerically the same person that he was yesterday, he would
judge that his physical structure is the same as it was yesterday, and he would reply in the
affirmative. Indeed, the theory of personal identity is at its core an affirmation that one can look
back on the experiences of days past and state with certainty, “It was I who underwent those
experiences, and not anybody else.” For instance, Alfred can recall what he had for breakfast
yesterday and assert, “I ate eggs and drank orange juice; it was not just some other person very
similar to me who sat at my table and had that meal.” Now, however, suppose that today I
truthfully inform Alfred that while he was sleeping last night, I snuck into his room, recorded the
exact position and state of all the atoms that make up his body, and then destroyed his body.
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Subsequently, I made an exact duplicate of the original Alfred based on the blueprint that I
recorded (let us assume that physicalism is true and that all mental and “soul-like” human
properties are just bodily properties, meaning that this duplicate must have come complete with
all of his memories, beliefs, hopes, and fears) and inserted this new Alfred into the original
Alfred’s bed. Then it seems clear that the person I am talking to today is actually Alfred’s
duplicate, and that the real Alfred is now dead. Even though the new Alfred remembers what the
old Alfred ate for breakfast yesterday just as if the new Alfred had been the one eating it, the new
Alfred is only a few hours old and therefore could not have been the one eating the meal. In
short, although the two Alfreds are psychologically continuous, they are spatiotemporally
discontinuous. So if I were to ask Alfred again whether he is numerically the same person that he
was yesterday, he would be forced to admit that he is not.
Suppose even further, though, that I build a robot to continue sneaking into Alfred’s room
on a nightly basis. Each night, the robot enters his room and flips a coin; if the coin comes up
heads, then the robot kills the existing Alfred and constructs a duplicate in his place, and if the
coin comes up tails, then the robot lets the existing Alfred live and leaves without doing
anything. There is no memory chip built into the robot, so no record ever remains of whether the
coin lands heads or tails. Finally, I inform Alfred about the robot and its routine. Now, every
morning, Alfred is in doubt about whether he survived the night or whether he is just a newly
minted duplicate. After all, in every case, Alfred feels as if he has lived a full life from childhood
to adulthood; every morning, he glances down and sees the scar on his arm that he remembers
receiving in sixth grade, and he looks in the mirror and sees the haircut that he recalls getting a
month ago. But it is a matter of chance whether his physical body is a few hours old or a few
weeks old; he will never be able to judge how long he has actually existed. So when I ask him
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whether he is numerically the same person that he was yesterday, he can never give me a
definitive answer. In fact, no one in the world can definitively say whether Alfred is old or new
at any particular point; no amount of evidence could possibly support or refute the persistence of
Alfred’s personal identity.
In this state of affairs, does Alfred have personal identity at all anymore? Alfred himself
would probably argue that he does. “Even though nobody knows whether I am a new duplicate
or an old one, not even me,” he might say, “one of two things will still happen tonight: either I
will survive, or the robot will kill me. If I survive, then I can rightly look forward to the positive
experiences I will have tomorrow, knowing that it will be me who will experience them, and I
can rightly fear the negative experiences, knowing that it will be me who will have to undergo
them. But if I die tonight, I will completely cease to think and be conscious. Although a duplicate
of me might enjoy tomorrow’s positive experiences, I cannot anticipate them, because I will not
be experiencing them, and similarly, although a duplicate of me might suffer negative
experiences tomorrow, I do not have to worry about them, because I will be dead by the time
tomorrow rolls around. The distinction between these two cases is not trivial; to me, it is of the
utmost importance!”
Alfred’s argument is a compelling one, and I find myself instinctively inclined to agree
with him. It is only with significant effort that I can make myself step back and consider the two
cases from a broader perspective. In the first case, the case in which Alfred survives, someone
who refers to himself as “Alfred” will enter the bedroom at night, and someone who refers to
himself as “Alfred” and who feels both psychologically and spatiotemporally continuous with
last night’s Alfred will leave the bedroom in the morning. In the second case, the case in which
Alfred dies and is replaced by a duplicate, exactly identical conditions will hold: someone who
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refers to himself as “Alfred” will enter the bedroom at night, and someone who refers to himself
as “Alfred” and who feels both psychologically and spatiotemporally continuous with last night’s
Alfred will leave the bedroom in the morning. The fact that the two Alfreds in the second case
are actually spatiotemporally discontinuous – that they are two different physical human beings –
is completely irrelevant, because the only two entities in the scenario that could possibly
perceive that discontinuity (the old Alfred, and the duplicate-making robot) are incapable of
doing so. The old Alfred dies instantly in his sleep and thus has no chance to perceive the
discontinuity, and the robot has no memory and thus cannot meaningfully comprehend the
discontinuity, so the gap in Alfred’s physical progression through time goes completely
unnoticed. Therefore, from all conceivable points of view, the case in which Alfred survives and
the case in which he dies are exactly identical.
This conclusion gives rise to the surprising corollary that death is not necessarily the end
of personal identity. To dispute this corollary, though, one must maintain that the two
aforementioned cases are somehow different, and to claim that the two cases are different is to
introduce an unnecessary element into the scenario, an illusory “self” or “consciousness” or
“identity” that exists independent of psychological continuity. This “self” would apparently
remain intact when Alfred’s body survives, but it would disappear when Alfred’s body is killed
and duplicated. Such a distinction would suggest a physical or spatiotemporal basis for identity,
but what basis? Maybe the old Alfred ceases to exist when, after being incinerated by the robot,
his atoms are no longer arranged in the particular formation usually associated with his body. But
human bodies constantly lose, gain, and reshuffle atoms while breathing in oxygen, shedding
dead skin, and engaging in other normal physiological functions, so it would be absurd if the
structure of Alfred’s atoms were the source of his identity. Perhaps, as Theodore Sider suggests,
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a continuous series of persons – instantaneous freeze-frames of Alfred in time – must lead from
the going-to-bed Alfred to the waking-up Alfred for the two of them to be numerically identical;
in other words, perhaps there can be no gaps in Alfred’s physical “story.” But the sudden loss of
a body part like an arm or leg does not seem to be enough of a “gap in the story” to constitute a
loss of personal identity, and the robot could easily build a new Alfred by incinerating old body
parts step by step and replacing them with new ones, so this criterion does not seem sufficient to
define personal identity either. Maybe the old Alfred has a soul that does not transfer into the
robot-made Alfred and thus leaves the new Alfred devoid of identity. But the addition of a soul
to the scenario unnecessarily introduces even more mystery to the already mysterious and
dualistic concept of Alfred’s intangible “self.”
In light of these alternatives, the wisest move may be to acknowledge that the elusive
concept of personal identity is simply a powerful fiction, and that Alfred’s sensations of fear and
anticipation regarding the persistence of his “identity” are simply vestiges of this deeply-rooted
fiction. Parfit does not go quite so far in his own writing; rather, he suggests that personal
identity be treated as a spectrum, on which “versions” of a person whose mutual temporal and
psychological links are tight should be mapped closer together than “versions” that are more
temporally and psychologically distant. According to this construction of identity, Alfred may
indeed have reason to fear the possibility that the person eating his breakfast tomorrow morning
will not be spatiotemporally continuous with him. Yet in my opinion, there is also something
comforting in the idea that personal identity is nothing more than psychological continuity. After
all, for all I know, I could be just a day-old duplicate of a person named “Scott Daniel,” and I
might be killed tonight to make room for yet another duplicate. But if I arm myself with the
difficult revelation that what I habitually call “personal identity” resides completely within
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psychological continuity, I can fall asleep with the assurance that whatever psychological
continuation of me rises from my bed tomorrow, my “self” will survive the night.
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