Since, like everyone, I generalise from single examples, I expect most people have some older relative or friend who they feel has added some wisdom to their life - some small pieces of information which seem to have pervasively wormed their way into more of their cognitive algorithms than you would expect, coloring and informing perceptions and decisions. For me, this would most be my grandfather. Over his now 92 years he has given me gems such as "always cut a pear before you peel it" (make quick checks of the value of success before committing to time consuming projects) and whenever someone says "that's never happened before", finishing their sentence with "said the old man when his donkey died" (just because something hasn't happened before doesn't mean it wasn't totally predictable).

Recently, though, I've been thinking about something else he has said, admittedly in mock seriousness: "If I lose my mind, you should take me out back and shoot me". We wouldn't, he wouldn't expect us to, but it's what he has said.

The reason I've been thinking of this darker quotation is that I've been spending a lot of time with people who have "lost their minds" in the way that he means. I am a medical student, and on a rotation in old age psychiatry, so have been talking to patients most of whom have some level of dementia, often layered with psychotic conditions such as intractable schizophrenia, some of whom increasingly can't remember their own pasts let alone their recent present. They can become fixed in untrue beliefs, their emotional become limited, or lose motivation to complete even simple tasks.

It can be scary. In some ways, such illness represents death by degrees. These people can remain happy and have a good quality of life, but it's certain that they are not entirely the people they once were. In fact, this is a question we have asked relatives when deciding whether someone is suffering from early dementia: "Overall, in the way she behaves, does this seem like your mother to you? Is this how your mother acts?". Sometimes, the answer is "No, it's like she is a different person", or "Only some of the time". It's a process of personality-approximation, blurring, abridging and changing the mind to create something not quite the same. What my grandfather fears is becoming a rough estimate of himself - though again, for some, that re-drawn person might be perfectly happy with who they are when they arrive.

Why is this of interest to LessWrong? I think it is because quite a few people here (me included) have at least thought about bidding to live forever using things like cryogenics and maybe brain-download. These things could work at some point; but what if they don't work perfectly? What if the people of the future can recover some of the information from a frozen brain, but not all of it? What if we had to miss off a few crucial memories, a few talents, maybe 60 points of IQ? Or even more subtle things - it's been written a few times that the entirety of who a person is in their brain, but that's probably not entirely true - the brain is influenced by the body, and aspects of your personality are probably influenced by how sensitive your adrenals are, the amount of fat you have, and even the community of bacteria in your intestines. Even a perfect neural computer-you wouldn't have these things; it would be subtle, but the created immortal agent wouldn't completely be you, as you are now. Somehow, though, missing my precise levels of testosterone would seem an acceptable compromise for the rest of my personality living forever, but missing the memory of my childhood, half my intelligence or my ability to change my opinion would leave me a lot less sure.

So here's the question I want to ask, to see what people think: If I offered you partial immortality - immortality for just part of you - how rough an approximation of "you" would you be willing to accept?