Philosophers are at the opposite extreme of engineers, in that they
consider the practical details of their thought experiments to be
unimportant. I think the idea that Parfit is exploring in the term
"psychological spectrum" is how much one can change and remain the
"same" person. I think I'm the same person I was yesterday even though
my brain has changed physically and my mind has changed to reflect that
change. Even though it is very unlikely to happen in reality, it is easy
enough to imagine that the relatively minor physical/psychological
changes that have occurred in the past day are exaggerated, so that
instead of changing from me-yesterday to me-today, I change from
me-yesterday into Napoleon. The point is that this type of radical
change would be different in *degree*, not different in kind from the
type of change that occurs normally. One could even argue that turning
into Napoleon is not as great a psychological change as that experienced
by an infant growing up, or an adult who becomes demented in old age.
Given these examples, what criterion can one come up with which allows
that one is the "same" person over the course of one's life, but a
"different" person if one changes into Napoleon (not having been
Napoleon previously)?

That last question is the search for a criterion for personal identity.
Parfit's answer is that the question is misguided. There is no objective
truth regarding continuity of identity over time, or as the result of
teleportation, or brain duplication, or all the other processes which
may in future become reality. What matters to us is not continuity of
personal identity in some absolute sense, but the feeling of
psychological continuity. And the reason *this* matters to us is simply
that our brains evolved that way: those organisms that do not believe
they are the "same" individual from moment to moment or day to day and
hence have no regard for their future well-being would soon die out.

Stathis Papaioannou

-----Original Message-----
[mailto:[EMAIL PROTECTED] On Behalf Of Russell Standish
Sent: Wednesday, 24 May 2006 5:29 PM
Subject: Reasons and Persons

Several list members cajoled me into reading David Parfit's "Reasons
and Persons". So I braved our dragon infested library, and sourced a
copy. I can see why his book is relevant to this list, particularly
part 3 of his book "Personal Identity". It was a good recommendation -
I can certainly recommend this as one of the background readers - too
late it missed the cutoff for my book :)

However, there was one thought experiment that concerned me, and it
relates to his notion of psychological spectrum. We are to suppose
that it is possible to generate psyches in between our mind and that
of Napoleon Bonaparte, by progressively swapping in neurons from NB's

Since we have a number of closet computationlists here, I paraphrased
the thought experiment as what if we swapped the transistors in my PC
for that of a (old-style PPC) Mac. At first, there would be little
difference, and the machine would be indistinguishable from that of a
PC - save a few bugs (anyone remember the Pentium division
bug?). Similarly, at the other end of the spectrum, the machine would
be virtually indistinguishable from a Mac. But what about the machines
in the middle? Surely these machine would simply be
non-functional. Replacing PC transistors with Mac transistors would be
no different from simply disabling the PC transistors - eventually a
critical path would be severed, and the machine would be defunct.

No two human brains are wired identically - indeed our daily
experience updates the connections between our neurons. Gradually
replacing neurons in our brain by someone else's neurons would have
the same effect as simply removing neurons one-by-one. For a while,
there would be little noticable effect - brains are, after all quite
robust against damage. But eventually, and well before the magical 50%
mark I would suggest, the structural organisation of our brain would
be lost, and we'd lose consciousness.

Since quite a bit of Parfit's later arguments depend on this
psychological spectrum thought experiment, it seems some of his
identity issues aren't in fact problems at all. Anyone have a comment
on this, or is it all obvious philosophy 101 stuff that I missed.


A/Prof Russell Standish                  Phone 8308 3119 (mobile)
Mathematics                                    0425 253119 (")
UNSW SYDNEY 2052                         [EMAIL PROTECTED]

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