Damascene Philosophy

Thought ExperimentsXI

Would you step into the teleporter?

It scans you, destroys your body, and builds a perfect copy on Mars a moment later. The copy wakes up remembering stepping in. Did you travel, or did you die?

Reading time
4 min
Difficulty
Intermediate
Branch
Identity
First posed
Derek Parfit, Reasons and Persons, 1984, drawing on earlier science fiction
EarthMarsscan, transmit, rebuild
Scanned and destroyed on Earth, rebuilt from new matter on Mars. Travel, or death and a copy?

The setup

It is the future. You want to visit Mars. The journey by spaceship takes months, but there is a faster way. You step into a booth on Earth. It scans every cell of your body, recording the exact state of every molecule. Your body is then painlessly vaporised. The information is sent to Mars at the speed of light, where a replicator builds a perfect copy of you, cell for cell, from local materials.

A few minutes later, someone steps out of the booth on Mars. They look exactly like you, remember everything you remember, including stepping into the booth on Earth, and have your personality, plans and love for your family. They feel completely continuous with you. As far as they are concerned, they are you, and the trip was instant.

Derek Parfit, who discussed this case in Reasons and Persons (1984), asked: is this a way of travelling, or a way of dying and being replaced by someone who only thinks they are you? Would you use it?

Why it matters

The teleporter isolates the question of what matters in survival. Everything you value about your future (your memories, character, relationships, projects) continues on Mars. Only physical continuity is broken. If you would refuse to step in, you seem to think something else matters: a continuing body, or a soul, or an unbroken stream of consciousness.

It also connects to real questions about mind uploading, cryonics, and even ordinary life, since the atoms in your body are gradually replaced anyway.

Key terms

  • Psychological continuityOverlapping chains of memories, intentions, beliefs and character linking a person at different times. Many theories of personal identity are built on it.
  • Personal identityWhat makes a person at one time the same person as someone at another time.
  • Numerical identityBeing one and the same thing, as opposed to being exactly alike. Twins may be qualitatively identical; only you are numerically identical to you.
  • Branch-line caseDerek Parfit's variant of the teleporter in which the original survives for a short while alongside the replica, forcing the question of which, if either, is you.
  • What mattersParfit's phrase for whatever is actually important to us in caring about our own survival, which he argued is psychological continuity rather than identity itself.

The positions

Position I

It is travel

The person on Mars is you. Psychological continuity, with the right cause, is what personal identity consists in.

Position II

It is death

You die in the booth. The person on Mars is a new person who is exactly like you.

Position III

Identity does not matter

There is no deep fact about whether the person on Mars is you. What matters, psychological continuity, is fully preserved, so teleportation is about as good as ordinary survival.

Arguments and objections

It is travel

Best argument
Everything that makes you you, your memories, personality, intentions and relationships, continues uninterrupted. Physical continuity matters only because it normally carries psychological continuity. Here the information is carried another way. Refusing to step in is like refusing to send a letter by email because the paper is not physically transported.
Strongest objection
The branch-line case: suppose the machine malfunctions and fails to destroy your body on Earth. Now there is a you on Earth and a you on Mars. Both are psychologically continuous with the person who stepped in. They cannot both be you, since they are two people. So psychological continuity cannot be what identity is.
The reply
Some say identity holds only when there is no competitor (a 'non-branching' condition). Parfit thought this showed that identity is not what matters: in the branching case, you get everything that matters in survival, twice.

It is death

Best argument
Your body, including your brain, is destroyed. A different body is built from different atoms. That the new person has your memories does not make them you, any more than a perfect forgery of a painting is the original. The branch-line case shows this: when the original survives, everyone agrees the person on Mars is a copy. Why should destroying the original turn a copy into the original?
Strongest objection
If physical continuity is what matters, then gradual replacement of your atoms, which happens anyway, should worry you as well. And the view makes your survival depend on something you could never notice from the inside.

Identity does not matter

Best argument
Parfit argued that once you know all the facts about bodies, brains and psychological connections, there is no further fact about identity to discover. The question 'is it really me?' is empty. And what you care about in survival, that someone will carry on your life, love your family and finish your projects, is all there on Mars.
Strongest objection
Most people cannot shake the feeling that there is a further question: will I experience Mars, or will everything go black? Parfit's answer seems to deny the most pressing question of all.
The reply
Parfit admitted that he could not fully believe his own view at an emotional level, but found that when he did, he felt less afraid of death.

Who argued what

Where you stand

Would you step into the teleporter?

Choose one to see how people answer.

Variations

The branch line

You step in. The Mars copy is made, but a fault leaves your Earth body intact. A technician apologises: the fault has damaged your heart, and you will die in a few days. Your Mars copy is fine. Should you be comforted that 'you' will live on, on Mars? Parfit argued that the comfort is real, if smaller than ordinary survival.

Star Trek's transporter

In Star Trek, the transporter dematerialises people and rematerialises them elsewhere. In one episode, a fault creates two copies of Commander Riker, who go on to live separate lives. Fans and philosophers have long debated whether everyone who uses the transporter dies and is replaced each time.

Gradual teleportation

Suppose the teleporter replaces your cells one at a time with cells built on Mars, while you remain conscious throughout. Does that feel safer? If so, why should the speed of the process matter? See the Ship of Theseus.

Go deeper

Parfit's liberation

Parfit wrote that before he came to his view, he felt imprisoned in himself, as if his life were a glass tunnel through which he moved faster every year, with darkness at the end. After accepting that personal identity is not what matters, he said, the walls of the tunnel disappeared. He felt he lived in the open air, cared less about his own future, and cared more about the lives of others.

Related ideas

Question

Are you the same person you were at eight?

Almost every cell, belief and memory has changed. Something makes that child you. The question is what.

Question

Are you your memories?

John Locke argued that you extend exactly as far back as you can remember. The idea is powerful, and it has holes you could drive a carriage through.

Thought experiment

The Ship of Theseus

Replace a ship's planks one at a time until none of the originals remain. Is it the same ship? And if someone builds a second ship from the old planks, which one is the original?

Thought experiment

Could you upload your mind?

If every connection in your brain were scanned and run on a computer, would the program be you, a copy of you, or no one at all?

Thought experiment

Two minds in one head?

When surgeons cut the connection between the brain's hemispheres, each side began to act as if it had a mind of its own.

Question

Is the self an illusion?

Look inward for the one who is looking, and you find thoughts, feelings and sensations. You never seem to find the looker.

Further reading and sources

Open questions

  • What exactly would you be afraid of losing in the teleporter?
  • If your copy on Mars is happy and loved, does it matter whether it is you?
  • Does an uninterrupted stream of consciousness matter more than an uninterrupted body?

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