Teletransportation
A machine scans your body perfectly, destroys the original, and recreates you atom-by-atom on Mars. The person who arrives has all your memories and feels continuous with you. Is that person you?
Philosopher Derek Parfit used this scenario extensively in his 1984 book to argue that personal identity might not be what matters in survival. His conclusion was radical: what we care about isn't whether it's really 'us.' It's psychological continuity, and that can come in degrees.
Parfit, D. (1984). Reasons and Persons. Oxford University Press.
The intuition pump
Close your eyes. Imagine stepping into the scanner, feeling a brief static, and opening your eyes on Mars. The sun is smaller. Someone hands you a coffee. You remember stepping in. You feel like you. Are you?
Most people's first answer: yes, that's me. The person has my memories, my personality, my intentions. What else could identity require?
But does it matter that the original was destroyed?
Parfit's branching problem
Parfit sharpens the puzzle by introducing a variant: the original isn't destroyed. A malfunction leaves two copies, one on Earth and one on Mars. Both have equal claim to being you. But they can't both be you. You can't be in two places. So neither is you?
If destroying one copy would reveal that neither is you, then why does destroying one copy during normal teletransportation make the remaining person you?
What Parfit concluded
Parfit's answer was that the question "is it really me?" might be the wrong question. What matters for survival isn't strict numerical identity. It's psychological continuity: the chain of memories, intentions, beliefs, and character that connects your present self to your past and future selves.
A teleported copy who loses five minutes of memory is slightly less continuous than one who loses nothing. Identity might be a matter of degree, not a binary yes/no.