Williams gives us two versions of a thought experiment. One seems to confirm the psychological criterion by suggesting that it's possible for persons to switch bodies by having their psychological properties transferred from one to the next. The other suggests that this is misleading and that what we thought of as transferring psychological properties from one person to another was really just altering the psychologies of two separate people who never leave their own bodies.
What's spooky is that they're both ways of describing the exact same experiment.
Drawing a line in Williams's spectrum
You all are the first group I've had that didn't strenuously try to draw a line somewhere along the spectrum. That's why I was surprised and worried that I might have illicitly encouraged you to say nice things about the article on the grounds that I had said that I liked it. Fortunately, you convinced me that you didn't really care about what I like. Whew!
Nonetheless, here are some points about drawing a line that are worth noting.
First, the key question is: would I feel the pain of torture, after being subjected to the changes in a given step?
If you think so, then you think that you would be the same person, even after the relevant changes.
If you think not, then you think that you have either died as a result of the changes (or that you've moved to a different body).
Second, you have to explain the difference the step makes. Once you've picked a step at which you don't think you'd feel the torture, try to explain why you think you wouldn't exist (if it's i - v) or why you'd exist somewhere else (vi).
But you're not done after doing that. You have to explain why you would be dead at this step but not the previous step.
It's no good to protest "But without X, Y, and Z, I'd be dead and so I'd have no reason to fear the pain of torture at all." In the previous step, you were imagined to have been deprived of X and Y. The step you think is the crucial step just involves an additional loss of Z. So you have to explain why the loss of Z makes all the difference, not why the loss of X, Y, and Z together makes the difference.
Questions of value and questions of identity
Williams thinks you could still feel pain after everything that is distinctive about you as a person is stripped away or changed. One might wonder if that wasn't missing the point about personal identity: what makes me a distinctive or unique person?
I think that's a deep point.
What I get out of Williams is something like the inverse of this point. All of my character traits, memories, and other distinctive psychological features can change and I will remain the same person. Proof: I can undergo these changes and I will still feel the changed person's pains. That's a sure indicator of my identity: if I can feel a person's pain, then I am that person. Most of the psychological characteristics that distinguish me from other people are just qualities: I don't need them in order to remain numerically the same person.
But why would I care about mere numerical identity if I lose my character, personality, and memory?
Well, I would. If I knew that a person was going to suffer great pain, it makes a great deal of difference whether he is numerically identical with me or not. It's the difference between my being in a great deal of pain and someone else's being in a great deal of pain. That's a significant difference!
But we rarely care about a person's mere numerical identity. We care about a person's qualities.
That's right. And that's why I think that there's a gap between the logically sufficient conditions for personal identity and the reasons why we find particular persons valuable (this is a point I made in discussing Quinton).
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