What did Parfit argue about identity and the teletransporter?

Derek Parfit argued that there is no deep further fact that makes a person at one time the same person as someone at a later time. He used teletransporter thought experiments to suggest that what matters in survival is Relation R, meaning psychological connectedness and continuity such as memory and personality, and that being destroyed and replicated is about as good as ordinary survival. [1] [2]

In Parfit's story, a person enters a Teletransporter on Earth. A scanner destroys the body and brain while recording the exact state of every cell. The information is sent by radio to Mars, where a Replicator builds a body and brain exactly like the original from new matter. The person who wakes on Mars remembers the life that came before and is in every other way just like the traveller. [2] [3]

Parfit notes that people who think about such cases divide. Some say the traveller wakes on Mars and that teletransportation is simply the fastest way to travel. Others say the traveller would be making a terrible mistake, because the person who wakes would be a mere Replica. [3]

Parfit then changes the machine so that it does not destroy the original. The blueprint still reaches Mars, but the original body survives with a damaged heart and will die in a few days. He talks to his Replica by video link. Here most readers feel that the Replica is a different person, since the two can talk to each other, and the original will die while the Replica lives on. A related version lets the machine make many replicas, all claiming to remember entering it. [2] [4]

Even if the Replica is not him, Parfit disagrees that the prospect is grim. In his account, being destroyed and replicated is about as good as ordinary survival. [2]

Parfit took these cases to support a deflationary view. Our existence involves our bodies, our deeds, our thoughts and other physical and mental events, not a separately existing entity. Identity over time consists of Relation R, with the right kind of cause, provided it does not branch into two future people. He also held that identity is not always determinate, so some questions such as whether one is about to die can be empty. [2]

His most distinctive claim is that identity is not what matters. Relation R matters, even when one person is R-related to two others, as in fission cases. There, the two successors cannot both be identical with the original because identity is transitive, yet Parfit held that having two such survivors is as good as, or even better than, being identical with one. He also added, more controversially, that the cause of Relation R could be any cause. [5] [2]