This paper argues that it has been widely assumed by philosophers of science that the cumulative retention of explanatory success is a "sine qua non" for making judgements about the progress or rational preferability of one theory over another. It has also been assumed that it is impossible to make objective, Comparative judgements of the acceptability of rival theories unless all the statements of both theories could be translated into a common language. This paper seeks to show that both these (...) dogmas are mistaken; that progress without cumulativity and comparability without commensurability are both viable. (shrink)