Beginning in the 1970's, a more focused discussion of the requirements of rational belief change has taken place in the philosophical community. Work by Isaac Levi (1977, 1980) and William Harper (1977) has been particularly important. In 1985, Carlos Alchourrón, Peter Gärdenfors, and David Makinson presented a formal model of belief change that is now known as the AGM model of belief change. Their work set a new standard for formal precision in this area. A rapid development of formal models of belief change has taken place in the 1990's, but it has not been matched by a sufficient amount of philosophical reflection on these models or on their relationship to issues in non-formalized epistemology.