Kuhn on incommensurability and theory choice

• Kuhn always drew an analogy between scientific progress and evolutionary development.
• Kuhn changed the analogy in his 1990s papers.
• The new analogy shows why scientific development isn’t harmed by incommensurability.

The incommensurability of two theories seems to problematize theory comparisons, which allow for the selection of the better of the two theories. If so, it becomes puzzling how the quality of theories can improve with time, i.e. how science can progress across changes in incommensurable theories. I argue that in papers published in the 1990s, Kuhn provided a novel way to resolve this apparent tension between incommensurability and scientific progress. He put forward an account of their compatibility which worked not by downplaying the negative consequences of incommensurability but instead by allowing them to reach their natural end: a process of specialisation. This development in Kuhn’s thought has yet to be properly recorded but it is also interesting in its own right. It shows how a robust version of incommensurability—one which really does have severe negative consequences for scientists’ capacity to perform comparative evaluations of incommensurable theories—need make no puzzle of the progress of science.