* no longer compute ∫x²dx
* remember the capital of France; and
* recall where they were born
Then they have crossed that threshold.
Personally, though I don't want to be killed. At least by my values, killing is not okay. I think there are much better things to do, such as:
- Being frozen in Antarctica, in case medical technology improves in a century or two to the point where I can be revived and cured.
- Go some place where I will surely die, but where I can still do some good (running a school for girls in Afghanistan when the US was there is a good example)
- If none of that is possible, do something interesting and dangerous. For example, if my brain is going anyways, experimenting with drugs and dying by drug overdose seems like a decent way to go.
And to be abundantly clear, that's my opinion and my values. I'm not trying to decide for anyone else. I don't think these sorts of decisions should ever be forced, mandated, or imposed.
Footnote: Thresholds are hard to define. I did have one relative with complete loss of short-term memory who seemed to be having a wonderful life. They were happy, friendly, and told grand kids wonderful stories from decades past. It helped that they had a very easy-going personality. On the other hand, they didn't know how they got to where they were, or what we were talking about five minutes ago. They also didn't have any recent memories (e.g. if a relative had been married within the past half-decade or so, they wouldn't recognize the spouse). There was a point where I'm pretty sure they wouldn't have passed any reasonable threshold, but they seemed to be living a very full life surrounded by a big family who loved them. On the other hand, if my brain was where their brain was, I'm pretty sure I'd be completely miserable.