This chapter examines the way contemporary psychiatry has shaped how humans understand selfhood. It considers the extent to which psychiatric nosology–via the Diagnostic and Statistical Manual (DSM)–has constrained the understanding of what it means to be human in Western culture. The chapter briefly discusses the influence of Descartes' substance dualism in medicine, as well as a brief history of the DSM published by the American Psychiatric Association. It addresses selfhood using insights from phenomenological philosophy and presents positive possibilities of a dimensional diagnostic model as an alternative to the categorical model. Despite no longer being philosophically de rigueur, dualist ontology continues to pervade everyday discourse. In order to understand the relationship between dualism and psychiatry, it is important to establish a basic outline of psychiatric diagnosis and its history. Phenomenology is an indispensable framework for both clinician and theorist alike.