Incommensurability
Kuhn pointed out that it could be difficult to assess whether a particular paradigm shift had actually led to progress, in the sense of explaining more facts, explaining more important facts, or providing better explanations, because the understanding of "more important", "better", etc. changed with the paradigm. The two versions of reality are thus incommensurable. Kuhn's version of incommensurability has an important psychological dimension; this is apparent from his analogy between a paradigm shift and the flip-over involved in some optical illusions. However, he subsequently diluted his commitment to incommensurability considerably, partly in the light of other studies of scientific development that did not involve revolutionary change. One of the examples that Kuhn used was the change in the style of chemical investigation that followed the work of Lavoisier on atomic theory in the late 18th Century as an example of incommensurability. In this change, the focus had shifted from the bulk properties of matter (such as hardness, colour, reactivity, etc.) to studies of atomic weights and quantitative studies of reactions. He suggested that it was impossible to make the comparison needed to judge which body of knowledge was better or more advanced. However, this change in research style (and paradigm) eventually (after more than a century) led to a theory of atomic structure that accounts well for the bulk properties of matter; see, for example, Brady's General Chemistry. This ability of science to back off, move sideways, and then advance is characteristic of the natural sciences, but contrasts with the position in some social sciences, notably economics.
This apparent ability does not guarantee that the account is veridical at anyone time, of course, and most modern philosophers of science are fallibilists. However, members of other disciplines do see the issue of incommensurability as a much greater obstacle to evaluations of "progress"; see, for example, Martin Slattery's Key Ideas in Sociology.
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