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In a now famous thought experiment, the philosopher Derek Parfit asks us to imagine a teleportation device that can beam a person from Earth to Mars. Rather than travel for many months on a spaceship, you need only enter a small chamber close to home and push a green button, and all the information in your brain and body will be sent to a similar station on Mars, where you will be reassembled down to the last atom. Imagine that several of your friends have already traveled to Mars this way and seem none the worse for it. They describe the experience as being one of instantaneous relocation: You push the green button and find yourself standing on Mars — where your most recent memory is of pushing the green button on Earth and wondering if anything would happen. So you decide to travel to Mars yourself. However, in the process of arranging your trip, you learn a troubling fact about the mechanics of teleportation: It turns out that the technicians wait for a person’s replica to be built on Mars before obliterating his original body on Earth. This has the benefit of leaving nothing to chance; if something goes wrong in the replication process, no harm has been done. However, it raises the following concern: While your double is beginning his day on Mars with all your memories, goals, and prejudices intact, you will be standing in the teleportation chamber on Earth, just staring at the green button. Imagine a voice coming over the intercom to congratulate you for arriving safely at your destination; in a few moments, you are told, your Earth body will be smashed to atoms. How would this be any different from simply being killed? To