Incommensurability and Vagueness

Abstract

Questions about incommensurability between theories have been a central topic in philosophy at least since the 1960's. Lately, perhaps, the topic has lost some of its edge: there is, I suspect, a fairly widespread agreement that, at least on some plausible ways of understanding what it is for two theories to be incommensurable, there will be some genuine cases of incommensurable theories though perhaps not as many as once was thought. But if the question of the existence of incommensurabilities has thus been largely settled in principle, I do not think that we have quite fully absorbed its implications in practice. There are many areas in philosophy, more I think than we commonly realize, in which issues about commensurability play a role. In particular, it often happens that, given a class of alternative theories about (in some suitably broad sense) a common subject matter, the question of how we are to interpret these theories turns out to be closely linked with the question, whether they will turn out commensurable with each other: interpret the theories in one way, and they will be commensurable; interpret them in another way, and they will not be. Now in such cases, the option of so interpreting the theories that they will turn out incommensurable is one that we should never dismiss out of hand: rather, knowing already that we will sometimes find cases of incommensurability, we should always be willing to consider the possibility that the case at hand is one such case. But this is what, I believe, we too seldom do: too often, we in effect treat the demand for commensurability as an unshakeable constraint on the sort of interpretation we are willing to consider. Thereby we sometimes overlook accounts that are better than the accounts we end up accepting. In this paper, I want to look at a few examples of the situation just described. Two will be cases in which the issues have for the most part come to rest, I think, and in favor of incommensurability; the last will be one in which discussion is ongoing, and I will advocate incommensurability. In addition to presenting examples, I will be making a few more general points about the sources of our