“Vera, a young victim of brain cancer, volunteers to undergo some new and experimental treatments. The treatments cure her cancer but they have profound side-effects. One effect is that she gradually loses her memories. She loses them in the order she acquired them, so that in thirty years, she will have a completely new set of memories. Another effect is that her personality gradually changes, so that in thirty years she will have an entirely new personality.
“It has been 40 years since the cancer treatments. The body that is continuous with Vera’s is now 55 years old and has a completely new set of memories and a completely new personality, not to mention a new name. If asked, ‘Melissa’ says she thinks of Vera like a grandmother, and knows about her only what people have told her. Does Vera still exist? It might be tempting to reply that there is a different person only in the weak sense. But this case involves changes much more radical than would normally occur. The changes are gradual, and there are enough similarities over any ten year period that the same person can be said to exist during that time. But by the time that 30 (let alone 40) years have passed there seems to be a different person, and not just in the weak sense.”