The meaning of incommensurability

In the last chapter we saw that in Kuhn's opinion scientific revolutions bring with them shifts in perceptual experience and changes in the world in which the scientist operates. These may be regarded as aspects of the more general phenomenon of incommensurability. The general idea of incommensurability is that the existence of changes in perception, world, standards of evaluation or in the meanings of key theoretical terms undermines traditional, Old Rationalist conceptions of progress as the accumulation of knowledge or as increasing verisimilitude. We have already considered the claim that changes in perception mean that there is no stock of theory-neutral observations that may be used for theory evaluation. In this chapter we will look at the parallel claim that shifts in meaning similarly preclude the possibility of a common measure of theories and prevent us from seeing progress as a matter of theories improving on their predecessors by getting closer to the truth.

The thesis that theories originating from different paradigms are incommensurable is one of the most keenly debated aspects of Kuhn's account of scientific change. The fact that this thesis has been taken to imply that such theories cannot be compared accounts for much of the controversy surrounding it. Yet it is important to be clear from the outset that incommensurability does not mean non-comparability; nor does it entail non-comparability in any trivial or straightforward way.