If you’re in the mood to go through a existential thought experiment, you should go read the above linked DailyMeh post. It’s a riff on how identity has a much stronger hold on us than we already suspect. Simen argues that dealing with “abnormalities” such as Pervasive Developmental Disorders are so difficult because if you accept that something is wrong with your mind, you have to accept that something is wrong with you. Thus Mind=Identity.
Human identity is layered, in a sense. We aren’t equally attached to every aspect of ourselves. Your foot is simply something you have, a part of you, surely, but not an essential aspect. Your mind, including your personality and your unique style of thinking and experiencing the world, on the other hand, is absolutely essential. You are your mind. You can replace almost every part of you and still be you, but not your mind. If I say, “I lost my foot in an accident”, the implication is that it’s still me, sans a foot, but if I say, “I lost my mind”, literally, the implication is that for a moment, I wasn’t myself, I was — or was acting as — a completely different person. They say you can’t step into the same river twice, everything being in flux and all, but some aspects of you are fairly constant while others change very much over time, sometimes over a short period of time. Personality — mind, cognitive processing — is one such aspect. At least, this is how human psychology works. Your mind is a constant through your life, and it is extremely attached to itself and its own ways. Human identity is built on this. It’s the essence of identity because our identity is for the most part an intuitive phenomenon, and we can change almost anything in radical ways without ruining identity, but the moment you start messing with someone’s mind, intuition says you’re making them into another person, not simply an altered version of themselves.