The subject of this essay is the dependence of evidential relations on background beliefs and assumptions. In Part I, two ways in which the relation between evidence and hypothesis is dependent on such assumptions are discussed and it is shown how in the context of appropriately differing background beliefs what is identifiable as the same state of affairs can be taken as evidence for conflicting hypotheses. The dependence of evidential relations on background beliefs is illustrated by discussions of the Michelson-Morley (...) experiment and the discovery of oxygen. In Part II, Hempel's analysis of confirmation and the contrasting model of theory acceptance provided by philosophers such as Kuhn and Feyerabend are discussed. It is argued that both are inadequate (on different grounds) and the problems addressed by each are shown to be more satisfactorily approached by means of the analysis developed in Part I. In Part III, it is argued that if there are objective criteria for deciding between competing theories, these cannot be simply that one theory has greater evidential support than another. Finally, some further methodological questions arising from the analysis are mentioned. (shrink)