It occurs to me that dementia is a process of death by subtraction. We lose stuff that's important to us - memories, for example, and language, and mobility, and awareness of who our friends are, and then of who our children are, and finally of who we are. It's mostly a slow process, though there are times when abilities or memories disappear overnight.
At the end of it all, before death arrives but once most everything else has gone, those observing the process are left wondering - who is this person? Without our memories, our ability to speak, and perhaps (though this is hard to determine) our thoughts, who are we? Our eyes may still shine brightly, we may smile, enjoy something to eat, hold a hand, but who are we?