Studies in History and Philosophy of Science Part A 119 (C):102203. 2026In this paper, we investigate the relationship between Popper's propensity interpretation of probability and Bohr's principle of complementarity. While Popper is often regarded as a philosophical antagonist of the Copenhagen interpretation, and Bohr never explicitly articulated his views on the nature of probability, we propose that the propensity interpretation can provide a coherent ontological foundation for Bohr's complementarity principle. This paper is primarily a rational reconstruction rather than a historical claim about Bohr's explicit commitments. We argue that both propensities and complementarity point toward a similar ontological picture—a move away from an object-based ontology of substances possessing intrinsic properties toward an ontology grounded in interactions, dispositions, and processes. According to this view, quantum phenomena are not predetermined but emerge from propensities inherent in the complete experimental arrangement. We also briefly consider how this framework relates to modern approaches of the decoherence-based program of emergent classicality. ( direct link )