When damage occurs to a person's brain, it changes them. If part of your brain was removed, you would lose that function of your brain. If each section of your brain was removed in turn, bit by bit your mental faclties would be taken away from you. Once there is no more brain left, you would be gone. Essentially, this is what happens with Dementia. This is why it's so devastating. By the end, they are not who they used to be. It's a form of gradual brain death.