This dissertation examines the central projects for a psychology of religion put forward in the late nineteenth and early twentieth centuries. It argues that the proponents of this subdiscipline were attempting to set up a new science of religion, one which they thought was radically different from the science(s) of religion(s) that had been created in the second half of the nineteenth century by thinkers such as Max Müller, C.P.Tiele, E.B. Tylor, or Albert Réville. The novelty of the psychology of religion was thought to reside in its identification of a primarily affective and pre-intellectual religious experience as the essence of religion and in the development of tools (e.g. questionnaires) and concepts (e.g. conversion, mysticism) with which to probe that experience. After a period of efflorescence in the first two decades of the twentieth century, the psychology of religion began declining in the 1930s. I argue that this decline was, in part, the result of an inability to maintain the theoretical integrity of the psychology of religion's topic of study, such that the discipline either became dissolved into general psychology or it became a private theology in its own right. Chapter 1 outlines the ways in which the sciences of religion were constructed by the aforementioned nineteenth century theorists, looking in particular at several Gifford lectures and at a number of prominent French historians of religions. Chapter 2 reconstructs the debates around the concept of conversion and its relation to the notion of feeling, examining the texts of American psychologists such as William James, E.D. Starbuck, or James H. Leuba. Chapter 3 examines the formation of the notion of 'mysticism' in the French-speaking psychology of religion, by looking, primarily, at the works of Théodore Flournoy, Henri Delacroix, Théodule Ribot, and Pierre Janet. Chapter 4 examines C.G. Jung's psychology of religion
