Revising Predictions

Making a prediction is essentially expressing a belief about the future. It is therefore natural to interpret later predictions as revisions of earlier ones and to investigate the notion of belief revision in this context. We study, both semantically and syntactically, the following principle of minimum revision of prediction: "as long as there are no surprises, that is, as long as what actually occurs had been predicted to occur, then everything which was predicted in the past, if still possible, should continue to be predicted, and no new predictions should be added." 1 I n t r o d u c t i o n W h e n we make a predict ion we select, among the several conceivable fu ture descriptions of the world, those t ha t appear to us to be most plausible. T h a t is~ making a predict ion is essentially expressing a belief abou t the future. It is therefore na tura l to interpret later predictions as revisions of earlier ones and to investigate the not ion of belief revision in this context . The not ion of rational belief revision is normal ly identified in the l i terature with the conservativity principle which states t h a t "When changing beliefs in response to new evidence, you should cont inue to believe as m a n y of the old beliefs as possible" (Harman, 1986, p. 46). This means t ha t if an individual gets new informat ion which is not inconsistent with her previous beliefs, t hen (1) she has to main ta in all the beliefs she previously had and (2) the change in her beliefs should be minimal in the sense tha t every new proposi t ion tha t she