Life as uniqueness.

The Thesis: Two identical brains in two identical worlds which are guaranteed to lead two absolutely identical lives, both in terms of internal subjective experience and in terms of their objective effect upon the world, are not different people.

1: Is personhood physical? (Spoiler: No)

If you think physical location defines a person, by that logic, if I were to move a step to the left, I would be a different person. (unless you have a very strange definition of a person where personhood is entangled with the environment). 

Does personhood requires physical continuity? What is physical continuity? Is it that the atoms in my body must be the same? Then I am a different person every 7 years. Every breath lessens me. Also, it implies a strange physicalism where a person resides in their atoms but not in other, empirically identical atoms. I do not see why replacing one carbon atom in a dendrite in my brain with an identical carbon atom makes me less me. Those who do are better off becoming shamans.

2: Is personhood causal?

Is it that a chain of causation must stretch from me to future me's? Why? No good definition of personhood or any phenomenon can depend on unrelated past states. A good definition must be empirical, based on the present configuration of the universe rather than past events. The alternative is to believe that two identical people in identical places with identical futures are different people only because of their pasts, To me this is insane. If you're not convinced, too bad.

(Rationality 101: Definitions of things that exist independently of our minds cannot be based on things that exist only in our minds, such as causation. Reality, like good code, is stateless)

3: Is personhood relational?

Some would argue that a person is only a person if their are other people they interact with. I don't think so.

1. Counterexample:If a meteor strike kills everyone except for me, I am still a person.
2. Even if humans can't develop into real people without socialisation, that isn't to say that is true of all possible lifeforms/people
3. A person is not entirely defined by their relationships. That is why if you replaced a human child with a dog, and treated the dog in an identical fashion, it would not grow into the same person.
4. There is a deeper problem with this definition, which is that it treats people as black boxes and only looks at how they interact with the external world, ignoring any differences in internal states. This is wrong. Put me and my girlfriend in front of a computer terminal which is our sole access to the external world. Give us only one key on the terminal which sends hello to the person on the other end. We produce the same output, but that does not mean we are the same person. We have different values, thoughts and feelings.

3: What is personhood

Who you are is defined, loosely, by what beliefs, values and preferences you hold. More strictly, what kind of person you are is defined by your internal state. There is an N-Dimensional person-space and you are somewhere in it. One perspective is that you are a point in this space. The problem with this is that it means that every tiny change makes you a different person. A better perspective is that a certain region or cloud of that space is you, more so at the enter and less as you move further away. You gradually change over days and months depending on your moods, emotions, growth etc.. but are, in the short term, similar enough to yourself that you are effectively the same person.