 The revised belief state should not differ more from the original belief state \([K]\) than what is motivated by \([p]\). This can be achieved by requiring that the outcome of revising \([K]\) by \([p]\) consists of those elements of \([p]\) that are as close as possible to \([K]\). For that purpose, \([K]\) can be thought of as surrounded by a system of concentric spheres (just as in David Lewis’s account of counterfactual conditionals). Each sphere represents a degree of closeness or similarity to \([K]\).