Academics tend to look on "esoteric," "occult," or "magical" beliefs with contempt, but are usually ignorant about the religious and philosophical traditions to which these terms refer, or their relevance to intellectual history. In this book I tell the neglected story of how intellectuals since the Renaissance have tried to come to terms with a cluster of "pagan" ideas from late antiquity that challenged the foundations of biblical religion and Greek rationality. Expelled from the academy on the basis of Protestant and Enlightenment polemics, these traditions have come to be perceived as the Other by which academics define their own identity to the present day. The discussion is grounded in a meticulous study of primary and secondary sources, thereby taking the reader on an intellectual voyage from the fifteenth century to the present day, while asking what implications the forgotten history of exclusion has for established textbook narratives of religion, philosophy, and science.