This chapter introduces Jaspers' understanding of delusion with regard to the question of comprehensibility and incomprehensibility. After a historical introduction, the characteristic feature of derealization is discussed and related to what Bleuler called "double book-keeping." Then, the methodological relevance of phenomenology and the similarity between delusional states and phenomenological imagination and bracketing (as emphasized by authors such as Blankenburg) are discussed. Different stances taken toward the world already in the realm of the natural attitude can be described within the Schutzian framework of "multiple realities." This framework is analyzed with regard to the notion of double book-keeping. It shows that some elements of schizophrenic experience can be seen—in Heideggerian terms—to preserve and reveal something about human authenticity.