Would You Step In?
This is an experiment in a different kind of article. It asks you questions as you go, and what you read next depends on how you answer. There are eight paths through it and they all arrive at the same conclusion. You can scroll back up and change any answer to see where the other paths lead.
- - -
It is 2190 and the commute to Mars takes four minutes. You walk into a booth in Tulsa. A scanner records the position of every atom in your body, and the booth takes you apart in the process. The record is sent to Mars at the speed of light, where an identical booth builds an identical body out of local carbon. That person walks out remembering your breakfast, your passwords and the name of your first dog. They feel like they just blinked.
Millions of people do this every day. Nobody who arrives has ever complained.
TULSA MARS
+--------+ +--------+
| o | . . . . . . . . . | o |
| /|\ | ------ signal -----> | /|\ |
| / \ | . . . . . . . . . | / \ |
+--------+ +--------+
scanned, rebuilt,
taken apart walks out
Your ticket is paid for. Do you step in?You stepped in. That means you think you are a pattern: the arrangement matters and the particular atoms don't, in the way a song is the same song on a different speaker. On that view nothing is lost in Tulsa, because everything that was you is on Mars.
It is a tidy view, and the booth is about to test it.
Something goes wrong. The scan completes and the signal is sent, but the Tulsa booth fails to take you apart. You step out, still on Earth. On Mars, someone with your face is already ordering coffee. The operator is apologetic. Policy is one of each customer. The Tulsa original will be painlessly put to sleep tonight. You are the one in Tulsa.
You are consistent, and that is rarer than you might think. If you are the pattern, then the pattern is safe on Mars, and what happens in Tulsa tonight is no worse than throwing away a spare copy of a file.
But a pattern can be copied more than once.
The operator, relieved you are being so reasonable, offers compensation. The scan is still on file. They can build as many of you as you like, on Mars, Titan, anywhere. Each one will wake up certain that they are you.
A hundred of you, then. Each will wake up with your memories and immediately start becoming someone else: one takes a job on Titan, one gets married, one takes up the cello. In ten years they will disagree about politics.
Ask which of them is you and the question stops making sense. They all carry on from where you were. You have followed the pattern view to the end, and at the end the word "I" no longer points at one thing.
You said no, and it is worth asking why. If you are only a pattern, a hundred copies is a hundred times as much of you, and that should sound like good news.
Your refusal says that being you includes being the only one. That requirement does not come from the pattern. A pattern has no objection to being copied. You added the requirement because it matters to you.
You were happy to be taken apart when the story was told from outside. When you were the one standing in the booth with the evening ahead of you, the Mars copy became a stranger with your memories.
So it seems you are not the pattern after all. You are this particular unbroken stream of experience, and the person on Mars has one of their own.
Consider last night. For several hours there was no stream at all. No experience, no awareness, nothing it was like to be you. This morning a brain with your memories started up again and assumed it was the same person. Is that different from the booth?
So the brain is what carries you through the gap. The experience stopped but the organ that produces it kept working, and that physical continuity is the thread.
That is a defensible place to stand. Notice that you moved to get here. You began as a pattern, became a stream of experience when the pattern was copied, and became a brain when the stream turned out to have gaps. Each time the case changed, you chose a new thing to be.
That is an uncomfortable thought and you are in good company for having it. If the stream is what you are, and the stream stops every night, then the person who wakes up inherits your life in the same way the Mars traveller does. They get the memories and the conviction, and you have no way to check anything more.
You have got out of bed every morning so far regardless. Whatever it is that survives a night's sleep, it has always been enough for you.
You took the rocket. Six months in a tin can, because you think the booth is a suicide machine with good marketing. The person on Mars would have your memories, but you would not be there to have them.
On this view you are the body that is sitting here. It is a physical thing with a continuous history, and a copy of it is a different thing.
Years later you develop a degenerative brain condition. The treatment replaces neurons with artificial ones that do the same job. One neuron a day, over many years. You are awake throughout. You never notice a single change. At the end there is no original tissue left. Is the person who finishes the treatment you?
So the original material can go, as long as it goes slowly. What you care about is that there was never a break: every day's version of you was nearly identical to the day before, and awake to vouch for it.
Slowness is doing a lot of work in that answer.
The clinic improves the procedure. The same replacement, neuron for neuron, now takes a year. Then a month. Then an afternoon. The newest model does it in one second, all at once, while you sit in the chair. That last one is the booth with the two ends in one room.
There is a line, you say, between ten years and one second. Try to say where it is. Is a month safe and three weeks fatal? Any place you put it will look arbitrary, because nothing in the physics changes at that point. Only the speed changes.
That does not make you wrong to want a line. It means you will not find it by looking closer at the neurons. You are the one who has to draw it.
You have talked yourself out of your first answer. If gradual replacement is survival, and nothing changes as it speeds up, then the booth is survival too and the six months on the rocket were for nothing.
Notice how you got here. No one showed you a new fact about teleporters. You were shown that two things you believed did not fit together, and you chose which one to keep.
You are holding firm: the original matter is you, and the finished patient is a very good prosthesis for a person who is gone.
Then there is an awkward fact about the body you are defending.
Your body trades its matter with the world constantly. The water is replaced in weeks. Skin, blood and the lining of your gut are rebuilt over and over. Most of the atoms in you now were not there ten years ago. Nature has been doing the slow version of the treatment since you were born.
So it was never the matter. It is the process: one metabolism, running without interruption since birth, taking atoms in and letting them go like a whirlpool in a river.
That is a good answer, and it is a different answer from the one you started with. You boarded the rocket to protect a physical thing. You are now protecting an activity. An activity could in principle be picked up somewhere else, and you will have to decide whether that counts.
You have taken the hard road. If the matter is you and the matter changes, then the child in your old photographs is someone else, and so is the person who will retire on your savings.
Almost nobody lives as though that were true. You keep promises made by your earlier self and you save money for your later one. Something ties those people together well enough for you to act on it every day, and it is not the atoms.
Whatever path you took, look at what kept happening. Each case gave you all the facts. You knew where every atom went, which memories ended up where, and what each person would say when they woke. Nothing was hidden from you, and the question of whether it was "really you" was still open. You had to settle it yourself each time. You did it by consulting what you care about: your memories, your body, being the only one, not being interrupted.
That is the main point. Personal identity looks like a fact waiting to be found, something a good enough scan would reveal. It behaves more like a decision. The cases show which things you are unwilling to lose, and "me" is the name you give to that bundle. Two careful people can know every fact about the booth and still disagree about stepping in.
The useful question, then, is what you want to carry on: the projects you would want finished, the people you would want looked after, the memories you would want someone to have. The booth only makes you answer it out loud.