I was watching Me, Myself, and Irene yesterday (and then Fight Club), and while these are not science fiction movies they do raise an interesting question about personal identity. Where do people who are bipolar or who have a multiple personality disorder fit in the personal identity question? Jim Carrey's character develops a split personality to cope with stress. More importantly the two different personalities are only present at different times and act independently of each other. A similar situation develops in Fight Clud with Tyler Durden. He has a split personality and while one is "awake" and doing things, the other is asleep and has no control over the other's actions. Furthermore, the two personalities have little knowledge of each others actions. Does a person like Tyler Durden have a single sense of personal identity or does the person have two individual identities inhabiting the same mind/body/thing. Are the multiple identities tied to each other, doing the things the other side won't, thus "completing" the individual or are they independant of each other?