The nature of the self depends on at least four presuppositions, namely a subjective awareness of activity, a subjective awareness of a sense of unity of self over time, an awareness of a persisting and singular identity over time and, finally, a distinct sense of being separate from other material objects and other selves. In this chapter, I deal with two conditions, possession states and dissociative identity disorder (multiple personality disorder), both of which seem to undermine the presupposition that the sense of self is predicated upon a unified sense of self over time and a sense of a persisting and singular identity over time.