The Branch-line Case

Notes for November 21

Main points

We talked about Parfit’s Teletransportation example. Specifically, we spent a lot of time trying to describe what happened in what he called the “branch-line case.” In that case, a person is teletransported to Mars without being destroyed on Earth. What happens to the person who got into the teletransporter on Earth? Did the person go to Mars, stay on Earth, die, or go to both places at once? That’s what we wanted to know.


Cyrus had a new one for me. He said that the original person is both on Mars and on Earth in the branch-line case. I have to say that in years of doing this, I don’t think I have ever heard that one before.

Jane, Angela, and Pamela gave two reasons for doubting Cyrus’s answer.

  1. It would mean that one person is in two places at the same time.
  2. It would mean that one person can have two “streams” of consciousness that never interact with one another: the one on Mars (where the room is painted red) and the one on Earth (where the room is painted blue). So if you ask “what does he see” the answer would be something like “part of him sees a red room with no blue at all and the other part of him sees a blue room with no red at all.”

Cyrus’s suggestion would be odd. But it’s interestingly difficult to show that it has to be mistaken. Maybe a person can be spread over space like that. And we know that it’s possible to cause people to experience different streams of consciousness that cannot interact with one another. It happens when you sever the connections between the hemispheres of the brain. So maybe the person in the branch-line case is just an exaggerated version of that.

Parfit’s answer

Patrick (and I think Josh) were more attracted to the kind of answer that Parfit himself gives. Parfit thinks that the original person survives teletransportation: the person on Mars is a continuation of the original person. But the person on Earth is also a continuation of the original person.

But, of course, neither can be identical with the original person. That is because there are two of them who are not identical with one another. (In order to reach this conclusion, you have to reject Cyrus’s suggestion, of course.)

Parfit’s conclusion is that I can survive into the future even if no one in the future is identical with me. If I were to be the person in the branch-line case, I would not be identical with either the person on Mars or the person on Earth. But I would still survive, he believes.

Next time

I closed with a comparison of Williams and Parfit. Williams thinks that people are special. They either exist to experience things in the future or they do not.

By contrast, questions about the identity of objects can have indeterminate answers. There is just no saying whether a watch that is made up of, say, 70% new parts is the same as the original watch so there is no saying if the original watch could survive having 70% of its parts replaced. All there is to say is that 70% of the parts have been replaced.

Parfit thinks that Williams is wrong to say people are special. We’ll return to this next time when we talk about Parfit’s combined spectrum example.

Key concepts

  1. How the branch-line case works.
  2. Why it seems to show that you can survive into the future even if you are not the same person as anyone in the future.
    This page was written by Michael Green for Problems of Philosophy, Philosophy 1, Fall 2013. It was posted November 21, 2013.
    Problems of Philosophy