In his treatise on the epistemological foundations of historical research, Simmel broadened the Kantian understanding of a priori, reformulating it in the neo-Kantian fashion as signifying ‘relational forms [Verbindungsformen], serviceable to that peculiar capacity of the mind which classifies, defines, and stresses each given mental content in such a way that it can be moulded to fit the most diverse constitutive forms’.1 Challenging the strict dichotomy between abstract perspective and empirical content, he believed that the notion of a priori could be used in a much more flexible way than had been envisioned by Kant and include contents which are themselves empirically derived and yet function in an a priori fashion, serving as presuppositions for specific branches of knowledge, such as history. Following Simmel in this regard, I will conclude my book with a few remarks on the methodological foundations of the work, or what can be described as the a priori of intellectual history, especially the kind of intellectual history which focuses on the ideas of individual minds.