Multiple personality disorder and possession are not necessarily mutually incompatible disorders. There's some evidence that you can have both.

As far as I am concerned, virtually all psychological diseases have their origin in our conscious minds. And that is not what we are taught.

I do not think that everybody has to struggle. But to probably at least half of the people, it never seems to enter their minds that they might be engaged in a struggle or that there might be something to struggle with.

M. Scott Peck Quotes