This chapter outlines a strictly internalist account of the major theoretical developments in physical science leading up to and during the interwar period, and the ways in which these developments conditioned the formation of novel epistemological positions. As key examples, Werner Heisenberg and Niels Bohr can be seen to have represented a more conceptually liberal accommodation of these demands, Albert Einstein and Max Planck a more conceptually conservative one. But this dichotomy, while broadly valid as a descriptor of the central debate over the epistemological status of the new quantum theory, obscures both nuance and counterintuitive conjunctions in the process through which these prominent physicists articulated their philosophical arguments. The essential goal of this chapter is to establish four well-articulated epistemological concepts that either emerged as a direct outcome of physical-theoretical developments in this context or else were given substantively new levels of valorization and detail: (1) an indeterministic conception of causality; (2) the complementarity of equally necessary yet mutually incompatible world-pictures; (3) the possibility of knowledge that is objectively valid yet not grounded in certainty, and (4) the emergence of counterintuitive yet irreducible epistemic limits specifically where technology enables us to venture beyond the scalar domain of normal human experience.