Real and Assumed Information

We introduce a formal model of how individuals form beliefs by simplifying complex real information through assumptions. We demonstrate that such assumed information, while often necessary, can lead to systematic excess certainty. By modeling belief formation under model uncertainty, we show that gaining new perspectives can increase subjective uncertainty, contrary to traditional models. The paper explores implications for persuasion and disagreement, emphasizing how differing unacknowledged assumptions obstruct belief convergence. A companion paper extends the theory and provides empirical support for the predicted patterns of overprecision and disagreement.