Cognitive psychologists have recently identified alterations in perception and reasoning that contribute to the formation and maintenance of beliefs that happen to be delusional. Clinically significant delusions, however, are often deeply unusual. An account of their formation and maintenance must explain not merely how someone can come to hold false or uncommon beliefs, but also how someone can arrive at beliefs that seem profoundly improbable and even bizarre. This paper uses the philosophical concepts of the Bedrock and the Background to provide an account of that foundational, nonrepresentational, non–rule-governed, dispositional structure of everyday understanding that underpins both our perception and our reasoning. The central claim is that the formation and maintenance of delusions becomes intelligible once they are seen to reflect disturbances to this foundational dispositional structure. Different dimensions of the Background are described along with the delusions that might follow from distortions to these dimensions. We challenge the assumption of much recent cognitive psychological theorizing that delusions can be understood as explanations of unusual experiences or as the product of hasty reasoning. Finally, the implications of the theory for the therapy of the delusional patient are briefly discussed.