The impossibility of immortality.

Defining Immortality

A person who lives forever is immortal. This requires two things:

1. That they live.
2. That they continue to do so for an infinite period of time.

Let's deal with 1 first. Here's why defining living is important.  Let's say someone is cryogenically frozen and kept in stasis for the next 1000 years. When they wake up, have they lived 1000 years? If we freeze someone in the depths of space, does the fact that they could plausibly be revived in a trillion years mean they are effectively immortal? I'd say no. Suspended animation, keeping a person in a state where they are not running (people are software), does not qualify as being alive*. X years in stasis does not mean that you have 'lived' X years. Why? Because a person in stasis does not think, and a person who does not have conscious thoughts is not alive**&*. Living is consciousness, and immortality requires a person to exist and be conscious, even if that consciousness is interrupted by stasis.

Why is immortality impossible?

As a person lives, they change. Our experiences shape us and change us. They change us in the short term, making us feel and think differently but they also shape us in the long run, changing our beliefs and values, our behaviour and our hopes and dreams. And therein lies the rub. Immortality implies infinite experience. No matter how minuscule the change each new experience effects, a positive amount times infinity is infinite. After enough time passes in an physically immortal beings life, it is invariably no longer the same person. Thus, the person it once was no longer exists and has died.

Objections:

First, let's deal with internal objections, that is those which operate within my paradigm and do not challenge my definitions of personhood and/or life.

Experiences effect us less and less as we age.

The argument is that not all experiences change us. When we are born our early life experiences, how we are raised, what culture we are steeped in, may indeed shape us greatly but later experiences shape us less and less. (insert example here). Hence, if we were to live forever, our character would solidify until, at a certain point, we are not longer changed by our environment.

The problem with this objection is fairly simple. If it is the case that the amount by which new experiences shift our character, while eventually falling to very low levels, remains positive, then over an infinite length of time those experiences will still change us into a different people. If it is the case that new experiences cease to have any effect on us whatsoever, then at that point we are effectively dead anyway. We've calcified into a fixed state incapable of change. Just like a person kept in stasis, we are not dead but neither are we alive. (reword. weak argumentation)

Experience does not change "core" traits, only peripheral ones.

This is a strong objection. It goes like this. Not all our traits define us equally. Some, like our fundamental values, baseline behaviours and personality traits define us a great deal. Stripped of my beliefs, behaviours and character, I would not be me. Others, like our current emotional state or knowledge of specifics of the world around us, do not. While my mental street map of the city in which I live may change depending on which city I live in, that does not mean I am a different person.(use a better example). Hence, an immortals experience could indeed change the peripheral elements of their mind, but leave the core untouched.

Objection: something

Not all persons must have a internal state.

Just because humans have an alterable mind does not mean all persons must. What if our immortal was not changed by their experiences. What if they were an a-priori perfect game player, able to compete effectively without any adaptation. What if they were, to take it to an extreme, a giant lookup table with reactions to every possible series of external stimuli hard-coded into the?

I'm not sure, but my intuition here is that this kind of lifeform is not a person. Why? Let's go through the objection tree.

• A lookup table containing all possible reactions to all stimuli cannot exist in the physical universe.
• Irrelevant. 1: it's a thought experiment. 2: A standard mind can be represented as a look-up table if you know the inputs it will recieve. Even if you don't, it can be a look up table where some inputs lead to a different lookup table which replaces the baseline one.
• All minds must have internal states. No internal state = not conscious = not a mind.
• True. But why must that internal state be malleable?
• To account for a changing world.
• What if the world is static?
• What if the static model is powerful enough to account for the world without further changes/optimization?
• 
  
• Life = change. 

* They're not dead either, but that's a discussion for another article.

** There should be separate words for alive in the sense of a living organism and alive in the sense of a person/conscious being continuing to exist. i.e: I can suffer brain death and cease to exist, but my body, the organism which hosts me, can continue to live.