This chapter focuses on the intellectual and political career of Kurt Riezler. Among the era’s foremost philosophers of politics and history, Riezler had an unusually deep understanding of contemporaneous developments within the philosophy of physics. His early forays into epistemological writing during the late Kaiserreich produced the concept of “calculated risk,” which tied together problems of knowledge and geo-strategy. During the Weimar-era phase of his career, he retreated from politics and doubled down on his engagement with scientific epistemology, in particular the philosophical problems posed by quantum mechanics. When Riezler finally returned to political-theoretical writing, after fleeing the Nazi regime to the United States, the refinements to his thinking produced by his long and careful study of contemporary scientific epistemology show themselves in a masterful series of essays. For Riezler, the essential point he came to express during this mature period was that epistemology matters for the philosophy of history (and by extension, politics) not because it determines ideology but because it indicates which avenues are open or closed for practical reason.