My body is part of me, as is my mind. Whichever part of myself remains, is me. If I am brain-dead, the body on life support is me. If only my mind remains, that's me. If they are seperated, but alive in one way or another, then each of those parts are also me

Though, for the record, I'd rather not be a brain in a jar nor hooked up to machines to breath. In both of those cases, given the choice, I'd chose death

As far as how little remains of me (or a thing) even if all that remains of me is a single cell. That's still me.

I'd take it even further than that, actually because I've given this way too much thought in the past. I don't have the mental fortitude to type it all out atm, but: I will happily argue that you are me, and I am you, and we are all temporary parts of a greater whole, operating as individuals on borrowed time with borrowed resources.

It depends on how much you want to zoom in/out. At a certain point one becomes the same as the other, like soup. Still, that soup wouldn't taste the same without it's individual ingredients, and each spice has it's own flavor- even if there's so little of it left, that no one can even taste it