The paper introduces precise definitions of the inevitabilist and contingentist positions. On this basis, it reveals the analytical structure and some inherent difficulties of the contingentist/inevitabilist issue. It then discusses some of these difficulties, relying on the thought-experiment of a ‘divided physics’. Along the way, it investigates the kinds of differences that two alternative physics should present, if they are to give rise to non-benign forms of contingentism.Keywords: Contingency; Inevitabilism; Incommensurability; Theory-comparison; Scientific realism; Constructivism.

Whereas there is much discussion about Thomas Kuhn’s notion of methodological incommensurability and many have seen his ideas as an attempt to allow for rational disagreement in science, so far no serious analysis of how exactly Kuhn aims to account for rational disagreement has been proposed. This paper provides the first in-depth analysis of Kuhn’s account of rational disagreement in science—an account that can be seen as the most prominent attempt to allow for rational disagreement in science. Three things will (...) be shown: First, we find not one, but two accounts of rational disagreements in science in Kuhn’s writings: one stemming from methodological incommensurability and one stemming from Kuhn-underdetermination, which are not only fundamentally different—the first purports to explain how disagreeing scientists can nevertheless be rational, while the second attempts to show how rational scientists can nevertheless disagree—but appear to be incompatible with each other. Second, I will assess both accounts. Kuhn’s account from methodological incommensurability is not convincing since it cannot explain rational disagreement in science. Whereas, on the other hand, Kuhn’s account from Kuhn-underdetermination allows for rational disagreement, his argument why we should accept it is not convincing. Third, I present a tentative sketch of an alternative to Kuhn’s account that emphasizes the fallibility of epistemic justification in order to show that Kuhn’s argument founders. In sum, the paper shows that focusing not on the muchly debated consequences of methodological incommensurability, but on Kuhn’s treatment of rational disagreement gives new insight into the adequate interpretation of his thought as well as the cogency of his ideas. (shrink)