This book sketches a unique intellectual portrait of the Dutch scholar Gerardus van der Leeuw (1890-1950) as a historian of religions and a phenomenologist, largely based on private archives and correspondence. The reader encounters a broad contextual investigation into his work, and into the network of scholars in Europe during the interwar period, a time of serious political turmoil. Van der Leeuw, influenced by Lucien Lévy-Bruhl’s concept of ‘primitive mentality’ and Karl Jaspers’ idea of understanding, created his very own structural-psychological phenomenology. His approach had considerable influence worldwide, notably on the work of Mircea Eliade, but it also raises serious questions about religionism, comparison and demarcation. The book provides a critical analysis of Van der Leeuw’s work, and engages in discussions on modes of thought, on the refutability of knowledge, and on the substantiation of comparison. It considers why and how to demarcate science from pseudo-science in the academic study of religion.