By means of a theoretical introduction and fourteen case studies, this dissertation formulates and validates its overarching thesis: that religious thought in the pre-modern world of Islam has at its core a trans-denominational disposition and therefore requires a historiographical approach that systematically and methodically transcends the boundaries of individual religious communities. Irrespective of the religious community, religious thought is the result of a constant and open-ended effort to understand, organize, and explain the manifold and changing manifestations of one’s own religion. The present dissertation understands these endeavours as an intellectual practice that has an irreducibly trans-denominational and transcultural complexion, as it is in constant exchange with other intellectual practices and decisively shaped by these transactions. In a first step (I.), the introductory essay presents various reasons why historical accounts of religious thought tend to remain within the confines of a particular religion or denomination, thus conveying the deceptive impression that religions are self-referential and self-contained entities. This tendency towards navel-gazing has been bolstered by the formation of disciplinary segmentation in Western academia, which has assigned a separate specialist discipline to each of the major religious traditions. At best, these disciplines engage in scattered exchanges with disciplines assigned to other religious traditions and occasionally resort to a comparative approach. In a second step, the basic components of trans-denominational religious thought are presented based on action-theoretical models that have been adopted from transcultural theories. Following the lead of transcultural theories, religious thought is understood as an aggregate of intentional and unintentional transactions that are marked by the characteristics of agents (II.1.), the constitution of commodities (II.2.), places and contexts (II.3.), and the type and mode of the (trans-)action (II.4.). The third part of the introductory essay (III.) presents the fourteen case studies included in this dissertation with reference to the aforementioned components of trans-denominational religious thought by clarifying which of them is relevant to each case study. The fourth part (IV.) traces precursors for the emergence of a trans-denominational approach to the history of religious thought within the humanities at European universities and outlines processes of disciplinarisation that hampered this development. Until well into the nineteenth century, ‘oriental’ religions were primarily studied from the vantage point of Christian theology and closely linked to the study of ‘oriental’ languages, each of which was assigned to a specific religious tradition, while at the same time being subordinate and auxiliary to the study of the biblical languages. The gradual abolition of admission restrictions for Jewish university teachers over the course of the nineteenth century generated a necessary condition that furthered the emergence of rudimentary trans-denominational approaches to religious thought. Such developments were dealt a heavy blow prior to World War II, not least because of the employment ban imposed on Jewish government employees. In recent decades, efforts to promote a trans-denominational approach to the history of religions and religious thought have again gathered pace thanks to the adoption of new methodological approaches from the historical sciences, the social sciences, and anthropology, which put an emphasis on transcultural relationality, entanglement, interconnectedness, and interaction. The final section of the introductory essay outlines the basic points of a heuristic framework that structures the trans-denominational approach to the history of religious thought. The fourteen case studies that follow the introductory essay are divided into three thematic clusters in accordance with their thematic thrust. The nine studies in the first cluster deal with various aspects of the Jewish Muʿtazila. The Muʿtazila was a dominant strand of speculative thought during the ʿAbbāsid period that has typically been presented as an Islamic school of thought. Jewish Muʿtazilism is not only a reflection of the fact that Jewish religious thought was at the time decisively shaped by the socio-cultural milieu of the Islamic world, but also makes plain that Jewish champions of Muʿtazilī thought actively contributed to advancing the intellectual profile of this school of speculative thought. Their works therefore constitute an invaluable source for drawing a more nuanced picture of Muʿtazilī thought. The second thematic cluster comprises four case studies that demonstrate the trans-denominational character of religious thought during the heyday of Copto-Arabic literature in the thirteenth and fourteenth centuries. During this period, the transformation of religious thought was driven by Coptic clerics and intellectuals whose family members held key positions in the Ayyubid state administration and thus enjoyed privileged access to the canon of knowledge of the educated elite. Based on the reception of the works of Maimonides and Fakhr al-Dīn al-Rāzī in major works by Coptic authors, the studies in this cluster exemplify that the renewal of religious thought in Coptic Christianity was largely negotiated through terms and concepts fleshed out in non-Christian literature. The third thematic cluster explores cross-connections between the thought of major representatives of kalām and Galenic literature during the ʿAbbāsid period. Generally, these two knowledge traditions have been associated with two sharply separated epistemic communities that functioned independently from each other and in mutual ignorance of one another. In stark contrast to this assumption, my study points to social and intellectual contexts that provided recurring opportunities for the exchange of ideas between two discrete bodies of knowledge. In addition, the relationship between kalām and the Galenic tradition is reassessed and resituated in the light of late antique points of contact between patristic and Galenic literature. Using the example of the concepts of ‘desire’/‘aversion’ and ‘pleasure’/‘pain’ in Muʿtazilī literature, the second part of the case study seeks to demonstrate that specific views of the eminent physician Abū Bakr Muḥammad bin Zakariyyāʾ al-Rāzī prompted intra-Muʿtazilī debates. The emphasis on the trans-denominational condition of religious thought, which constitutes the common thread of the fourteen case studies included in this dissertation, is grounded in the methodological hypothesis that the dynamic history of religious thought is best explored and understood in light of a trans-denominational approach, as it reveals important facets of religious thought that remain hidden to a self-referential, mono-denominational historiography.
