This dissertation seeks to clarify and articulate a trajectory in Germanic Jewish thought, beginning with the work of Moses Mendelssohn and ending with Martin Buber; the two concepts mapped by this trajectory are the in-between [zwischen] and singularity [die Singularität]. These ideas are developed in light of their philosophical context, and brought into dialogue with contemporary thought. Singularities are individuals that are not particulars, meaning: not particular instantiations of either a general or universal concept. Singularities cannot be grasped in terms of their differences and similarities to other things. Singularity is what makes something what it is, independent of its predicates or containment in a universal. The in-between is the ontological space that allows for an encounter between singularities, such that this encounter helps constitute the singularities meeting. In this sense, the in-between is the chief conceptual support of the idea of singularity. Because singularities are autonomous and cannot be defined by their differences from other singularities, the space where singularities encounter each other cannot be thought of as a container without transforming the singulars into particulars. This dissertation takes the modest goal of formalizing and clarifying these concepts, preparing the concepts of the in-between and singularity such that they can one day be used in the study of religion. I suggest that these concepts are useful insofar as they allow us to take a middle path between theology and reductive analysis, viewing the transcendent claims of religions in a manner that is at once sympathetic and critical.