Both Umberto Eco and Charles S. Peirce have been concerned with the notion of background knowledge. Eco refers to background knowledge as the encyclopedia; Peirce’s term of reference is collateral experience. The aim of this article is to investigate the degree to which these two concepts are comparable. We focus on one major metaphysical issue, viz. the fact that Eco defines collateral experience, which is the first step in any process of cognition, as private, whereas Peirce, as a realist, would never accept the concept of private thoughts, feelings, etc. We suggest that freeing collateral experience from its nominalistic nomenclature makes possible a comparison and synthesis of Eco’s and Peirce’s conceptions when seen from the perspectives of their cognitive type, nuclear type, and molar content.