 Kuhn developed and refined his initial idea over the following decades, repeatedly emphasizing that incommensurability neither means nor implies incomparability; nor does it make science irrational (e.g. Kuhn 2000 [1970], 155ff.). He focused increasingly on conceptual incompatibility as manifest in the structural differences used to classify the kinds whose relations are stated in laws and theories, such as chemical elements and biological species (Kuhn 2000, see especially chs. 3, 4, 5, 10 &amp; 11). He used incommensurability to attack the idea, prominent among logical positivists and logical empiricists, that comparing theories requires translating their consequences into a neutral observation language (cf. Hoyningen-Huene 1993, 213–214). In the late 1990s, he explicated incommensurability in terms of ineffability, emphasizing that it became possible for scientists to make and understand certain new statements only after a particular theory had been introduced (in the older vocabulary the new sentences are nonsensical), just as it only becomes possible for historians to understand certain older statements by setting aside current conceptions that otherwise cause distortions (Kuhn 2000 [1989], 58–59; 2000 [1993], 244). Such ‘taxonomic incommensurability’ results in translation failure between local subsets of inter-defined terms due to the cross-classification of objects into mutually exclusive taxonomies. This can be distinguished from ‘methodological incommensurability’, according to which there is no common measure between successive scientific theories, in the sense that theory comparison is sometimes a matter of weighing historically developing values, not following fixed, definitive rules (Sankey and Hoyningen-Huene 2001, vii-xv). This makes room for rational disagreement in theory comparison, as scientists may apply different values (such as scope, simplicity, fruitfulness, accuracy) in evaluating and comparing particular theories, so that theory choice is not unequivocally determined throughout the scientific community.