Main points
Parfit argues that questions about personal identity do not
necessarily have determinate answers. He does so by asking us to
consider a spectrum of cases in which a person changes in a variety of
ways.
Let’s call the person before the changes are made A and the person
after the changes have been made B. After considering each change, we
can ask “Is A the same person as B?” In many cases, the answer will be
“yes” or it will be “no.” The conclusion Parfit hopes to establish is
that there are some changes for which it is equally appropriate
to say “yes” or “no” or even “yes and no.”
The Combined Spectrum
Parfit starts by repeating Williams’s argument. That’s what his
“psychological spectrum” involves: a series of minute changes to a
person’s mind. If you would survive one such change, it seems, you would
survive them all, even to the point where your mind is completely
different. That suggests personal identity is the same thing as bodily
identity: you stay with your body even though your mind has been
completely altered.
But things are not so simple. What if we did something similar with
your body? In the “physical spectrum,” we imagine a series of minute
changes to a person’s body. If you could survive one such change, it
seems, you would survive them all, even to the point where your body is
completely different. That suggests personal identity is the same thing
as psychological identity: you stay with your mind even though your body
has been completely destroyed.
An inquiring mind might ask “what if we combined them?” OK, what if
we did that? Imagine a series of minute changes to your body
and mind. The person at the far end of the spectrum would have
a body and mind like that of Greta Garbo.
Would you be the same person as the Greta Garbo person? Parfit does
not think so. There is nothing left of you: both body and mind are gone.
That tells us there is something fishy about these spectrum
experiments.
Most of us would have felt pretty good to have come that far. But
Parfit had far too original and searching a mind to have stopped
there.
We have two points that seem pretty solid.
No single minute change to mind and body is enough to make the
difference between survival and death.
The person on the left side of the Combined Spectrum is not the
same as the person on the right side.
That’s a bit of a head scratcher. Some of the changes on the
Combined Spectrum must be enough to kill the person on the left side. We
established that this person is gone by the time you get to the right
side. But no particular change is enough to make the difference between
remaining alive and dying. What do we make of this?
Parfit’s proposal is that there are a range of changes for which it
would be indeterminate whether the original person survived the
change. That means it would not be definitely true that the original
person was identical with the changed person and it would not be
definitely false.
An analogy with colors might help. Take two colors: red and orange.
There is a spectrum here too. Start on the left: it’s definitely red.
You can add a minute amount of yellow to red and it will still be red.
If you add a lot of yellow to red, the color will be orange. The color
on the right side is definitely orange. But somewhere in the middle,
there are colors that are sort of red and sort of orange. They aren’t
either red or orange, mind you. So you can’t say they are the same color
as red. And you can’t say they are not the same color as red. There is
no fact about whether the colors in this zone are red or not. The only
facts are the proportion of red and yellow in them.
Here’s a way of summarizing Parfit’s view. There isn’t anything for a
person to be other than a body or a mind. But both physical and mental
continuity can come in degrees. So there have to be cases in which it is
indeterminate whether the original person survives some changes. By
analogy, the amount of red in a color comes in degrees and that is why
there can be cases in which it is indeterminate whether a given color is
the same as red.
There is, of course, an alternative. If personal identity consists in
the identity of an immaterial soul that cannot be divided, then we would
not be driven to this conclusion.