Consequently, comparison between theories will not be as straightforward as the standard empiricist picture would have it, since the standards of evaluation are themselves subject to change. This sort of difficulty in theory comparison is an instance of what Kuhn and Feyerabend called ‘incommensurability’. Theories are incommensurable when they share no common measure. Thus, if paradigms are the measures of attempted puzzle-solutions, then puzzle-solutions developed in different eras of normal science will be judged by comparison to differing paradigms and so lack a common measure. The term ‘incommensurable’ derives from a mathematical use, according to which the side and diagonal of a square are incommensurable in virtue of there being no unit that can be used to measure both exactly. Kuhn stressed that incommensurability did not mean non-comparability (just as the side and diagonal of a square are comparable in many respects). Even so, it is clear that at the very least Kuhn’s incommensurability thesis would make theory comparison rather more difficult than had commonly been supposed, and in some cases impossible.