At the end of the lecture last time, I mentioned Kuhn's view that different paradigms are incommensurable, i.e., that there is no neutral standpoint from which to evaluate two different paradigms in a given discipline. To put the matter succinctly, Kuhn argues that different paradigms are incommensurable (1) because they involve different scientific language, which express quite different sorts of conceptual frameworks (even when the words used are the same), (2) because they do not acknowledge, address, or perceive the same observational data, (3) because they are not concerned to answer the same questions, or resolve the same problems, and (4) they do not agree on what counts as an adequate, or even legitimate, explanation.

Many authors took the first sense of incommensurability (linguistic, conceptual) to be the primary one: the reason scientists differed with regard to the paradigms, and there was no neutral standpoint to decide the issues between the two paradigms, is that there is no language (conceptual scheme) in which the two paradigms can be stated. That is why the two sides "inevitably talk past each other" during revolutionary periods. Kuhn seems to assume that because two theories differ in what they say mass is like (it is conserved vs. it is not, and exchangeable with energy) that the term "mass" means something different to the two sides. Thus, there is an assumption implicit in his argument about how theoretical terms acquire meaning, something like the following.

The two sides make very different, incompatible claims about mass.

The theoretical context as a whole (the word and its role within a paradigm)
determines the meaning of a theoretical or observational term.

The two sides mean something different by "mass."

On this interpretation of Kuhn, the two sides of the debate during a revolution talk past each other because they're simply speaking different (but very similar sounding) languages. This includes not only the abstract terms like "planet" or "electron," but observational terms like "mass," "weight," "volume" and so on. This contrasts with the older ("positivist") view espoused by Carnap, for example, which held that there was a neutral observational language by which experimental results could be stated when debating the merits (or deficiencies) of different candidates for paradigm status. The two scientists might disagree on whether mass is conserved, but agree on whether a pointer on a measuring apparatus is in a certain position. If one theory predicts that the pointer will be in one location, whereas other predicts it will be in a different location, the two cannot both be correct and so we then need only check to see which is right. On Kuhn's view (as we are interpreting him here), this is a naive description, since it assumes a sharp dichotomy between theoretical and observational language. (On the other hand, if his view is simply holistic, so that any change in belief counts as a conceptual change, it is implausible, possibly vacuous.)

This interpretation of Kuhn makes him out to be highly problematic: if the two groups are talking about different things, how can they really conflict or disagree with one another? If one group talks about "mass" and another about "mass*" it's as if the two are simply discussing apples and oranges. Theories are on this interpretation strictly incomparable. Newtonian and relativistic mechanics could not be rivals. This is important since Kuhn himself speaks as though the anomalies are the deciding point between the two theories: one paradigm cannot solve it, the other can. If they are dealing with different empirical data, then they're not even trying to solve the same thing.

The problem becomes more acute when you consider remarks that Kuhn makes that seem to mean that the conceptual scheme (paradigm) is self-justifying, so that any debate expressed in the different languages of the two groups will necessarily be circular at some point. That is, there is no compelling reason to accept one paradigm unless you already accept that paradigm: reasons to accept a paradigm are conclusive only within that paradigm itself. If that is so, however, what reason could anyone have to give up the old paradigm? (In addition, it cannot be literally correct that the paradigm is self-justifying, since otherwise there would be no anomalies.)

An alternative view would reject the thesis that the incommensurability of scientific concepts or language is the primary one; rather it is the incommensurability between scientific problems. That is, if the two paradigms view different problems as demanding quite different solutions, and accept different standards for evaluating proposed solutions to those problems, they may overlap conceptually to a large degree, enough to disagree and be rivals, but still reach a point at which the disagreement cannot be settled by appeal to experimental data or logic. "When paradigms change," he says, "there are usually significant shifts in the criteria determining both the legitimacy of problems and of proposed solutions...." "To the extent ... that two scientific schools disagree about what is a problem and what is a solution, they will inevitably talk through each other when debating the relative merits of their respective paradigms." The resulting arguments will be "partially circular." "Since no paradigm ever solves all the problems it defines and since no two paradigms leave all the same problems unsolved, paradigm debates always involve the question: which problems is it more significant to have solved?"

On this view, what makes theories "incommensurable" with each other is that they differ on their standards of evaluation; this difference is the result of their accepting different exemplars as definitive of how work in that discipline should proceed. They are, indeed, making different value judgments about research in their discipline.

How are different value judgments resolved? One focus of many critics has been Kuhn's insistence to compare scientific revolutions with political or religious revolutions, and with paradigm change as a kind of "conversion." Since conversion is not a rational process, it is argued, then this comparison suggests that neither is scientific revolution, and so science is an irrational enterprise, where persuasion--by force, if necessary--is the only way for proponents of the new paradigm to gain ascendancy. Reasoned debate has no place during scientific revolutions. Whether this is an apt characterization of Kuhn's point depends on whether conversion to a religious or political viewpoint is an irrational enterprise.

Kuhn himself does not endorse the radical conclusions just outlined; he does not view science as irrational. In deciding between different paradigms, people can give good reasons for favoring one paradigm over another, he says; it is just that those reasons cannot be codified into an algorithmic "scientific method," that would decide the point "objectively" and conclusively. There are different standards of evaluation for what counts as the important problems to solve, and what counts as an admissible solution. For pre-Daltonian chemistry (with the phlogiston theory of combustion), explaining weight gains and losses was not viewed to be as important as explaining why metals resembled each other more than they did their ores. Quantitative comparisons were secondary to qualitative ones. Thus the weight gain problem was not viewed as a central difficulty for the theory, though an anomalous one. Things were different in the new theory, in which how elements combined and in what proportions became the primary topic of research. Here there was a common phenomenon on which they could agree--weight gain during combustion--but it was not accorded the same importance by both schools.

Under this interpretation, much of what Kuhn says is misleading, e.g., his highly metaphorical discussion about scientists who accept different paradigms living in different worlds. Kuhn sometimes seems to be making an argument, based on Gestalt psychology, of the following form.

Scientists who accept different paradigms experience the world in different ways; they notice some things the others do not, and vice versa.

The world consists of the sum of your experiences.

Scientists who accept different paradigms experience different worlds.

Some of his arguments depend on the assumption that to re-conceptualize something, to view it is a different way, is to see a different thing. Thus, he speaks as if one scientist sees a planet where another saw a moving star, or that Lavoisier saw oxygen whereas Priestley saw "dephlogisticated air."

This is an incommensurability of experience; it is dubious, but this does not detract from the very real incommensurability of standards that Kuhn brought to the attention of the philosophical, historical, and scientific communities.