Did the teleporter travel you, or kill you?
Imagine a teleporter that scans your body down to the atom, destroys it, and beams the data to Mars, where a replica is built. The Martian you has all your memories and feelings. But are they actually you, or did you die in the scanner while a perfect clone was born? Philosopher Derek Parfit used this thought experiment to argue that personal identity is not about physical survival.
The Blueprint and the Beaming Device
Imagine stepping into a booth on Earth to travel to Mars. The mechanism is simple and flawless: a scanner records the exact state of every cell, molecule, and atom in your body, down to the precise synaptic connections in your brain. As the scan completes, your original physical form is painlessly disintegrated. Simultaneously, the complete dataset is beamed across space to a receiver on Mars. Using local materials, an identical biological structure is assembled atom for atom.
When the replica wakes up on the Martian surface, they feel entirely continuous with the person who entered the booth on Earth. They recall breakfast, remember their childhood, possess identical skills, and have the exact same plans for the afternoon. From the perspective of outside observers, the process looks like rapid, seamless transportation. But beneath the surface lies a fundamental puzzle of philosophy: did you actually travel to Mars, or did you die on Earth while a brand-new person who merely believes they are you took your place?
The Branch-Line Malfunction
In his 1984 book Reasons and Persons, British philosopher Derek Parfit exposed the hidden tension in this scenario by introducing a famous variation known as the branch-line case. Suppose the teleporter undergoes a minor technical change: it scans your entire body and beams the blueprints to Mars, but due to a mechanical adjustment, it does not destroy the original body on Earth.
You step out of the booth on Earth, completely intact, and the technician informs you that the Mars replica has been created successfully. A moment later, the technician mentions that the scanner caused a fatal heart defect in your Earth body, and you have only a few days left to live. You are looking at the screen, watching your duplicate on Mars begin their new life, feeling everything you feel and thinking your thoughts.