This is important, because a standard conception of the transition from classical to relativistic physics is that although Einstein’s theory of relativity supersedes Newton’s theory, what we have is an improvement or generalization whereby Newton’s theory is a special case of Einstein’s (to a close approximation). We can therefore say that the later theory is closer to the truth than the older theory. Kuhn’s view that ‘mass’ as used by Newton cannot be translated by ‘mass’ as used by Einstein allegedly renders this kind of comparison impossible. Hence incommensurability is supposed to rule out convergent realism, the view that science shows ever improving approximation to the truth. (Kuhn also thinks, for independent reasons, that the very ideas of matching the truth and similarity to the truth are incoherent (1970a, 206).)