Identity
and the Copying of Minds
Alexander R. Pruss
Department of Philosophy
Georgetown University
November 20, 2006
Draft
of Work in Progress: I am not entirely convinced by the arguments here, but
I think they’re worth thinking about.
Assume for simplicity that human mental states are constituted by brain states (if dualism holds, copying of brain states may need to be replaced with copying of soul states). According to psychological continuity theories of personal identity, if the personality and memories of a human person A were copied into the brain of B while the brain of A were destroyed, and no other copies were made, then A would survive in the B-body and would be identical with the post-operative person occupying the B-body. By considering the mechanics of copying in a case where A and B have qualitatively identical brains, I will argue that the conclusion is false, and hence the psychological continuity theories that imply this identity claim are false.
Psychological
continuity theories typically distinguish connectedness from continuity. Simplifying by restricting myself to
the case of memory (the forward-looking case of intentions emphasized by Parfit
(??ref) is analogous), if t2>t1, a person x at t1
is connected to a person y at t2 if and only if y has a first-personal quasi-memory of something that
befell x at t1.
If t2<t1, we say x at t1
is connected to y at t2 if and only if y at t2
is connected to x at t1, and by default we deem x at t
to be psychologically connected to x
at t if x exists at t. A quasi-memory is just
like a memory, minus the requirement, if there is one(??ref), that one can only
have a first-personal memory of something that one has oneself done.
Psychological continuity, then, is the transitive relation generated by
psychological connectedness: x at
t1 is psychologically
continuous with y at t2 provided there is a chain of
psychological connectedness in between.
Psychological continuity theorists think that, at least absent
branching(??ref), psychological continuity is necessary and sufficient for
personal identity.
My argument will not work against a psychological continuity theorist who believes that quasi-memories need to be connected to the quasi-remembered events by a causal connection of such a sort as can only exists within a single brain. The endorsement of mind-copying thought experiments seems to be, however, one of the main reasons for accepting psychological continuity theories over brain and bodily identity theories (??ref).
I shall assume I am dealing with a
psychological continuity theorist who thinks that a substantive connection
needed between a quasi-memory and what it is the quasi-memory of. A swamp-man in China arising
coincidentally right after I perish and with apparent memories of what I did
but with no causal connection to me will not count as having quasi-memories of
what I did. However, the
connection between a quasi-memory and the quasi-remembered event is not so
tight as to rule out mind-copying thought experiments. This, I take it, is a description of
the view of a typical psychological continuity theorist.
Suppose that we have a way of
describing a brain state as a finite sequence of N bits, where a bit is something that can have value 0
or 1. If brains can be described
by mathematical physics, we can indeed do this, at least up to any required
degree of precision, and presumably there comes a point at which we have enough precision to capture the mental state that
supervenes on the brain state (e.g., in capturing the state of a digital
computer, we may need not note the exact voltage level in a given area, but
simply to note whether the voltage is close to the value representing zero or
close to the value representing one).
Now I will describe a process
whereby the data of the brains a and b of two people are copied into a single output brain c. Call
the two initial persons A and B, respectively.
We thus read in the data from the brains a and b
bit by bit, then feed the corresponding bits from each of these brains into a
machine that takes two bits, one from a and one from b, and
outputs one bit. We take the
output bit and impose it on c,
while destroying the parts of the original brains that have already been
read.
I will call the machine that
computes the one output bit from the two input bits “the Combiner”. The Combiner, I posit, has two
settings, the X setting and the Y setting.
The setting is put in ahead of time, e.g., by flipping a switch or in
some other way, and works as follows.
If the Combiner is given two bits that are equal, i.e., both zero or
both one, then it immediately outputs the same bit value. If the two bits are not equal, the
machine checks its setting. If the
setting is X, then the bit that
came in from a is put out. If the setting is Y, then the bit that came in from b is put out.
The
following two claims seem to hold given psychological identity theory:
(1) If the Combiner
setting is X, then A but not B is identical
with the post-operative person with brain c.
(2) If the Combiner
setting is Y, then B but not A is identical
with the post-operative person with brain c.
These claims hold for any two persons A and B
whose mental states supervene on brain states in such a way that the brain
states can be described with N
bits.
Now
consider a special case. In this
special case, the brains a and b are qualitatively identical but numerically distinct. Maybe A and B
are twins, one of whom grew up on Earth and the other on Twin-Earth, or maybe
they are the result of a previous fission. In any case, however this may have happened, A and B
have qualitatively the same brain state at the beginning of the procedure while
being distinct persons.
I
now claim that who survives the operation and who is identical with the c-brain person after the operation does not depend on whether the Combiner setting is X or Y. The reason for this is simple. Because the brains a and b
have the same state at the beginning of the procedure, the bits read from each
are always the same, and the Combiner simply outputs the common value. The Combiner never checks which setting
it has when it gets a pair of equal inputs. We could imagine the Combiner’s setting being set not by
flipping a switch, but by putting the inscription “X” or “Y”
in a sealed envelope, with the Combiner having manipulators and sensors which
would open the envelope as soon as it gets two bits that differ, so it could
find out how to process them. But
the envelope is never in fact opened when the brains a and b
are in the same state. And whether
a person survives and inhabits a given brain surely does not depend on what had
been written in a sealed envelope whose contents in fact have no causal
influence on anything outside the envelope.
If
this is correct, then in the case at hand, the identity facts for A and B
are the same regardless of the Combiner setting. But according to (1)
and (2),
the identity facts for A and B differ depending on the value of the Combiner
setting. Since (1)
and (2)
follow from the psychological continuity theory of personal identity, it
follows that the psychological continuity theory is false.
I.
Tu quoque. This is a puzzling case, and like many such it will be
puzzling for every theory of personal identity.
This
objection is simply off-base. There
is nothing problematic about this case on body-identity, brain-identity,
animalist or dualist views. The
first three of these views imply that (1)
and (2)
are both false, regardless of whether the states of a and b
are the same or not. Dualism
implies that the cases are underdescribed because we failed to say what happens
to the immaterial soul(??Swinburne).
II.
No-branching requirements. Psychological identity theorists
typically (??refs) make an exception for cases of branching, and say that
psychological continuity is sufficient for identity absent branching.
Branching can take the form of fission or fusion. Here we have fusion, the objection
goes, and hence (1)
and (2)
are false.
But
we do not have fusion here. If the Combiner has setting X, then it is a machine that copies the state of a into c,
with the state of b being
counterfactually irrelevant, while if it has setting Y, then it is a machine that copies the state of b into c. In neither setting is the Combiner a
machine that fuses the
states. Now it is true that in the
case where a and b are qualitatively identical, the person with brain c ends up being qualitatively just like A and just like B. But, nonetheless, brain c is a copy of a, not a copy of b, given
setting X, and is a copy of b, not a copy of a, given setting Y, assuming “copy of” implies counterfactual
dependence. There is no fusion.
III.
Causal dependence. Suppose a and b
do have qualitatively the same state.
Then given setting X, the
output of the Combiner has no counterfactual dependence on b. But
nonetheless it has a causal
dependence on b in the sense that
the machine functions differently depending on the state of b. If the
bit coming from b were not the
same as the bit coming from a,
then the Combiner would have to have looked at the setting, and decided in
light of it. Thus the causal
functioning of the Combiner with setting X does depend on the state
of b, despite the lack of
counterfactual dependence of the output on b. And
this is enough to ensure that there is psychological continuity between B and the person with brain c. Given
that there clearly also is psychological continuity between A and the person with brain c, it follows that we do have a case of fusion—both A and B
are psychologically continuous with the possessor of c—and hence (1)
is false, the objection insists.
By the same token, (2)
is false, and hence there is no problem here for the psychological continuity
theorist.
But
the mere fact that b causally affects
the functioning of the Combiner
is of little importance. After
all, so do many other things, such as atmospheric changes in the vicinity of
the Combiner, vibrations in the table on which it is set, and brain-waves from
the person who turns the Combiner on.
As long as these things do not affect the output values, they are irrelevant to us here.
Let
us, however, consider the claim that there can be a causal connection between the state of b and the output of the Combiner in setting X.
Suppose that the nth bit
in the description of b is 1, and
so is the nth bit in the
description of a. One might then argue that the nth bit of the description of b’s being 1 together with the nth bit of the description of a’s being 1 causes the Combiner to output a 1. Thus the states of both a and b
are partial causes of the nth bit
of c being 1. If, further, a and b
have qualitatively the same brain state, as in the case I am interested in,
then this is true for every value of n, and so the state of a
is a partial cause of the state of c
and so is the state of b.
Hence, if being a partial cause is enough for quasi-memory-type connections, then A and B
are each psychologically connected with the resulting person who has brain c, and so by no-branching considerations neither A nor B
is identical with the c-brain
person. On the other hand, if
being a partial cause is not
enough, then neither A nor B is connected or continuous with the resulting person
who has brain c. In either case, it seems, the
psychological continuity theorist can deny each of (1)
and (2).
However
this overstates the importance of being a partial cause. Let us suppose that we have a machine
that simply copies bits from brain a to
brain c while destroying brain a, and does not read b at all, but the machine is powered by the electrical
activity of brain b. Then the states of brain b, insofar as they affect the electrical activity of b, do indeed enter into the causal story. But surely this is not enough to make
the person with brain c count as
connected with B. In the case where the Combiner is
powered by the electrical activity of b, there is no counterfactual dependence of the Combiner’s output on the
details of b’s brain state, and exactly the same thing is true in
the case of the Combiner with setting X. Likewise, the fact that b is nearby may jiggle some of the electrical
potentials in an a-to-c copy machine in a way that is insignificant
vis-à-vis the outputs of the machine, but this surely is not relevant to
questions of identity, again because of the lack of counterfactual dependence.
It
seems that counterfactual dependence is what matters for the causal aspect of
quasi-memories, and this is indeed how it should be. Someone who correctly quasi-remembers something also knows
it. But knowledge through memory
or quasi-memory involves precisely that kind of counterfactual dependence: this
dependence is what we need in order to ensure that it is not a coincidence that
the person has the apparent memory or quasi-memory. If the setting is X,
then were the states of a
different, those of c would be
correspondingly different, but no similar relation holds between the states of b and those of c. This seems to be the
right counterfactual dependence condition for quasi-memories and
quasi-intentions. Hence, (1)
holds if psychological continuity is the right theory of personal
identity. By the same reasoning,
so does (2).
There
is also a general response that can be made to objections that proceed by
insisting that in the special case where a
and b are qualitatively
identical, (1)
and (2)
are both false and in fact neither A nor
B is identical with the possessor
of c after the operation. Since (1)
and (2)
hold in all other cases given psychological continuity, we get the absurdity
that, say, with the Combiner set to X,
person A would survive the
operation if state of B’s were
slightly unlike that of A’s,
whereas A would not survive if B had exactly the same qualitative brain state, even
though the output of the machine would be qualitatively exactly the same
whether B was exactly or almost
exactly like A, and would have
exactly the same counterfactual dependencies.
IV.
Biting the bullet. The responses to the above objections
might teach one that what matters for the causal aspect of psychological
continuity is counterfactual dependence.
But now the road is open to denying the claim that the Combiner setting
is irrelevant to identity facts in the case where a and b
are qualitatively identical. Even
in that case, whether A is identical with the resulting person with brain c or whether it is B that is identical with that person depends precisely
on the setting of the Combiner, on the present objection. The Combiner setting never enters into
the causal story, but it is nonetheless is counterfactually relevant, and this
is what we need.
This
view is absurd for the reasons already noted. The contents of sealed envelopes which do not actually enter
into causal relations with anybody in the story do not determine facts about
personal identity in that story (of course the contents might be evidence for a fact about personal identity if, say, they are
a statement by an expert on some relevant question, but that is not what we are
talking about). We can vary the
example in many ways. Instead of a
sealed envelope, we can imagine that the Combiner when first faced with a mismatch
would ask a third party, Fred.
Then who, if anybody, is identical with whom would depend on facts about
Fred’s disposition to answer a hypothetical question. This is absurd.
V. Survival, not identity. Parfit
distinguishes survival from identity, and makes survival the important
feature. Perhaps, then, I can say
that in the case where a and b are qualitatively identical, both A and B survive, regardless of the Combiner setting. The case of qualitative identity is,
then, different from that where a
and b are not qualitatively
identical, but there is principled ground for the difference: it is because the brain of A has the same data as the brain of B that both can survive through one person with brain c.
But if the Combiner setting is,
say, X, then it is not clear what makes
it be the case that B survives
through the person with brain c. The mere fact that c happens to have the same mental states as B is insufficient, since that fails to connect these
mental states—it is just a coincidence that the possessor of c has the same mental states as B, since the counterfactual dependence is on the
states of A. If I were sentenced to death and found
out that there is a Twin Earth where there is a twin qualitatively identical
with me, but the histories of Earth and Twin Earth are about to diverge so that
the twin would survive, this would not assuage my egocentric concerns, though
it would make me glad that somebody would fulfill some of my plans, even if he
would not fulfill them because they were mine.
We
may now note that there is a further counterexample to any psychological
identity theorist who requires an appropriate counterfactual dependence of
quasi-memories on the quasi-remembered events. In this case, we have only two brains a and b
with qualitatively identical states—there is no separate output brain c. A
scanning machine examines each bit of data in a, and if that bit matches the corresponding bit in b, it leaves it be in a, but otherwise it imposes the data from b onto the data in a. All
the while, b is being
destroyed. Thus there is
counterfactual dependence of the post-operative state of brain a on the state of b before the operation, but no counterfactual
dependence of the post-operative state of brain a on its own pre-operative state.
Now
if A survives after the operation, it is
clear that a is A’s brain, since there is no other brain there for A to have after the operation. But the quasi-memories in a after the operation do not have the needed
counterfactual dependence on the events that had befallen A before the operation. Hence, if the psychological theory of personal identity
holds and if counterfactual dependence of quasi-memories on quasi-remembered
events is needed, it follows that A
can no longer exist after the operation.
But this is absurd, because the only thing that actually happened to A during the operation was that A’s brain was scanned. All
the other changes were merely counterfactual—brain a was put in danger of having the states of b imposed on it, but in fact no such imposition
happened. The intuition that A would not be affected by such an operation is very
similar to the intuition in Frankfurt-style arguments that one’s freedom is not
affected by the mere fact of a neurosurgeon’s standing watching one’s brain and
being ready to intervene in counterfactual situations.
Things are tough for the
psychological continuity theorist: I have just given an argument for why she
should deny the necessity of counterfactual dependence. But at the same time she should accept the necessity of counterfactual dependence
because causal dependence is not enough.
This is strong evidence against psychological identity theories of
personal identity.
The
possibility of qualitatively identical persons provides strong evidence against
typical psychological continuity theories. The reason for this appears to me to be that psychological
connectedness places too great an emphasis on qualitative identity rather than
on non-qualitative conditions, such as persistence of chunks of matter. Consequently there is probably a whole
slew of counterexamples to psychological continuity theories that can be
generated by using qualitatively identical persons. For instance, imagine scanning two such persons, destroying
the brains, and saving the scans on a hard drive for future restoration. However, suppose that the computer has
clever data compression software that detects identical bit-sequences, and
replaces them with a single sequence plus a note that the sequence is to be
duplicated. Thus, the backups of
the two persons got replaced with a backup of one, plus a note that there was
another file identical with that one.
Could one then recreate the two original persons from such a compressed
backup? If so, which person would
continue which? I leave it to the
imagination of the reader to generate other cases.
Are
cases of qualitative identity a problem for other theories of identity? Maybe. But at least the present problem is not, as we have seen in
the response to the first objection.