The scholastic controversy on probable opinions in the seventeenth century was one of the most extensive and acrimonious debates of the early modern era. Historiography has treated it as a quarrel over moral casuistry, but this underestimates its import. The scholastic preoccupation with the ‘use of opinions’ should be understood as a search for a general framework for dealing with reasonable disagreement between competent evaluators of truth claims (not only moral ones). In the early modern era, scholastic analyses as well as regulations concerning the prudent and legitimate use of opinions acquired an unprecedented scope and depth. For the first time in European intellectual history, detailed theories of reasonable disagreement emerged, based on explicit characterizations of competing probable opinions as reasonably tenable.