What are the criteria for what makes someone the person they are? A lot of changes we can go through leave us existing as the same people. We've changed, but we're still there as the people who have changed. Some things that can be done to us leave us no longer around, as much as we don't like to think about that. Most cases of both are uncontroversial. But that doesn't tell us what it is to be us, and philosophers raise lots of puzzles about what changes we might undergo without ceasing to be us.
Philosophers will answer these kinds of questions by talking about essential properties. An essential property is something necessary for a thing to be what it is. An essential property of a triangle is having three angles. If it somehow got a fourth angle, it would no longer be a triangle. The triangle would cease to exist. My having a beard is not an essential property. I can shave my beard, but I would still be around afterward, and I'd be exactly the same person I was beforehand. Which properties are essential is a matter of debate, however.
Some people, following Rene Descartes, hold that we have a part that's not physical at all but an immaterial mind or soul. This view, dualism, would say you need the same soul to be the same person. [Descartes' own view is that the immaterial mind is not just a part of him but is all of him. His body is just a place his mind occupies. Most dualists think rather that the mind and body are both parts of us.] It's not immediately clear with some changes (e.g. if you used a Star Trek transporter) if the same soul would be present afterward according to dualism.
Other people say you need the same body. If so, you die when your body dies, but you continue if your body is still alive, even if other things aren't present (e.g. a functioning brain). Some say you need the same brain. If so, you might end up with a new body if your brain gets moved to a new body.
Some might say you need psychological continuity, e.g. having a continuing set of beliefs, desires, hopes, fears, loves, character traits, and so on (which obviously get somewhat changed over time but only gradually and through a process where most of them continue). Some who hold this view have even suggested we could be converted to computer programs and survive that way. [John Searle questions this, as we've seen in his Chinese Room argument. Behavior as if you think isn't enough for genuine thinking.]
These same criteria come up in John Perry's A Dialogue on Personal Identity and Immortality. Is it possible to exist after you've died? Lots of people think so, so it must not be obviously absurd. Gretchen Weirob is about to die. She wants the slightest possibility that she'll continue to exist. What if someone in the future has all her characteristics at her death? No - it has to be her, not just someone exactly like her (e.g. an identical twin who somehow also had the same memories and exactly the same personality traits). That's the kind of identity we mean, not just exact similarity but really being Weirob herself. She can anticipate what she will do and look forward to it, because it's not someone else. It's never correct to anticipate doing something that someone else will do. That's just an imposter.
Consider the example of burning a Kleenex box. If you later say of something "this is the very same box of Kleenex", that seems absurd. Even if you reconstructed something exactly like it, it wouldn't be the same box. Consider the same with the Mona Lisa.
So how could I survive death? The next post will begin looking at the different personal identity views, how they answer that question, and the various objections to them.