This chapter situates Ludwik Fleck’s references to Lucien Lévy-Bruhl and Ernst Mach to clarify the distinctiveness of his proposed ‘comparative epistemology’. It examines Fleck’s engagement with Lévy-Bruhl’s sociological anthropology and Mach’s conventionalist history of science, showing how he draws on both traditions while departing from each in critical respects. Against Lévy-Bruhl, Fleck highlights the positivist assumptions underpinning the claim that scientific thought yields purely objective concepts. Against Mach, he critiques the internalist and biologically grounded account of scientific development for neglecting socio-cultural determinants. Fleck’s discussion of scientific facts does not stem from systematic engagement with Mach’s epistemology, but from a critical reading that exposes implicit assumptions about perception and facticity. Reconsidering Jerzy Giedymin’s insightful critique, the chapter contends that Fleck, despite mischaracterizing aspects of Machian thought, successfully identifies a blind spot in Mach’s conception of historical development. Central to this reappraisal is Fleck’s notion of ‘passive elements’ in cognition—relational constraints that emerge from interaction with what is cognized. Rather than reintroducing objective reality ‘through the back door’, these elements function as a check on arbitrary constructivism, anchoring cognition in resistance. Recent scholarship has revisited this neglected component of Fleck’s theory, emphasizing its relevance to epistemological models grounded in interaction and evolutionary adaptation. In light of these contributions, the chapter affirms Fleck’s continuing importance to debates on the historical, social, and cognitive constitution of scientific facts.