Kuhn's Incommensurability Thesis

What's the Argument?

Moti Mizrahi

1. INTRODUCTION

Since Thomas Kuhn first introduced the incommensurability thesis in 1962, it has undergone several transformations, even in Kuhn's own writings (Hoyningen-Huene 1993, 206–22). Basically, the thesis has been understood in at least two different ways:

Taxonomic Incommensurability (TI): Periods of scientific change (in particular, revolutionary change) that exhibit TI are scientific developments in which existing concepts are replaced with new concepts that are incompatible with the older concepts. The new concepts are incompatible with the old concepts in the following sense: two competing scientific theories are conceptually incompatible (or incommensurable) just in case they do not share the same "lexical taxonomy." A lexical taxonomy contains the structures and vocabulary that are used to state a theory (see Kuhn 2000, 14–15, 63, 92–97, 229–33, 238–39, and 242–44. Cf. Sankey 1997).

Methodological Incommensurability (MI): There are no objective criteria of theory evaluation. The familiar criteria of evaluation, such as simplicity and fruitfulness, are not a fixed set of rules. Rather, they vary with the currently dominant paradigm (see Kuhn 1962, 94; 1970, 200; 1977, 322, and 331. Cf. Sankey and Hoyningen-Huene 2001: xiii).

Kuhn's incommensurability thesis has generated an enormous literature. In this paper, I wish to examine the evidence that is supposed to support Kuhn's incommensurability thesis. This is an important task, I think, for the following reasons. First, some authors use the language of "discovery" when writing about Kuhn's incommensurability thesis. According to Oberheim and Hoyningen-Huene (2013), for instance, Kuhn "discovered incommensurability in the mid to late 1940s" (emphasis added). This "discovery" was then followed by a change in career paths. Using the "discovery" language in talking about Kuhn's incommensurability thesis gives the impression that incommensurability is a fact about scientific change (revolutionary change, in particular). But what if one does not think that incommensurability is a fact about scientific change? Are there compelling epistemic reasons to think that the incommensurability thesis is indeed true or probable? In other words, are there good arguments for the incommensurability thesis?

Second, and perhaps as a consequence of the first reason, it seems that the literature has focused more on exploring the implications of Kuhn's incommensurability thesis than on evaluating the arguments for the thesis itself. This is not to say that no one has disputed Kuhn's incommensurability thesis (see, e.g., Scheffler 1982). Arguing that a thesis is false, however, is not the same as arguing that there is neither valid deductive nor strong inductive support for a thesis.

In what follows, I focus on (TI) rather than (MI). Here is how I plan to proceed. In section 2, I argue that there is no valid deductive argument for (TI), since from the fact that the reference of the same kind terms changes or discontinues from one theoretical framework to another, it does not necessarily follow that these two theoretical frameworks are taxonomically incommensurable. In section 3, I argue that there is no strong inductive argument for (TI) because there are rebutting defeaters against (TI). That is, there are episodes from the history of science that do not exhibit discontinuity and replacement, as (TI) predicts but rather continuity and supplementation. If this is correct, then there are no compelling arguments in support of (TI). In section 4, I discuss and reply to an objection made by an anonymous referee of the journal Social Epistemology, where this chapter was originally published.

2. WHY THERE IS NO VALID DEDUCTIVE SUPPORT FOR (TI)

Is there deductively valid support for (TI)? That is, is there a valid deductive inference from reference change to incompatibility of conceptual content, and thus to (TI)? By appealing to the distinction between sense and reference, Scheffler (1982, 59–60) has already shown that variation of sense does not entail incompatibility of conceptual content. In that case, perhaps reference change is supposed to entail incompatibility of conceptual content, and hence taxonomic incommensurability. That is, "Distinct taxonomic structures (ones with different subsumption and exclusion relations) are inevitably incommensurable, because those very differences result in terms with fundamentally disparate meanings" (Conant and Haugeland 2000, 5; emphasis added). Indeed, in his later works, Kuhn talks about reference change as "redistribution" of members among "taxonomic categories" (Kuhn 2000, 30). Such "referential changes," according to Kuhn (2000, 15), show that "scientific development cannot be quite cumulative," that "one cannot get from the old to the new simply by an addition to what was already known," and that one cannot "describe the new in the vocabulary of the old or vice versa" (emphasis added). Furthermore, as Sankey (2009, 197) points out:

Analysis of the reasoning employed by Kuhn [...] when [he] argue[s] for [...] reference change reveals that [he] assume[s] that reference is determined by description.

Accordingly, I take it that a deductive argument for (TI) would run roughly like this:

(TI1) If competing theories were taxonomically commensurable, then terms would still refer to the same things in new theories (e.g., "mass" in Newtonian mechanics and "mass" in relativistic mechanics would have the same referent).

(TI2) Terms do not refer to the same things in new theories (e.g., "mass" in Newtonian mechanics and "mass" in relativistic mechanics do not have the same referent, and some terms, such as "phlogiston," are eliminated outright).

Therefore:

(TI3) Competing theories are taxonomically incommensurable.

In addition to "mass," another example that is supposed to illustrate how reference change is evidence for incompatibility of conceptual...