 A third taxonomy (see Brown 1991, chapter 2), which has not gone unchallenged (see Norton 1993b), is more limited than the first two insofar as it focuses largely on the class of those thought experiments that are taken to function in theory choice, which is the use of thought experiments that has been receiving most of the attention. According to this taxonomy, the main division is constructive vs. destructive and resembles Karl Popper’s distinction between apologetic and critical thought experiments. Popper actually distinguishes between three types of thought experiments: heuristic (to illustrate a theory), critical (against a theory) and apologetic (in favour of a theory) (see Popper 1959). His case in favour of a critical and against an apologetic use of thought experiments is very limited. He focuses exclusively on quantum physics and doesn’t really say much to address the primary epistemological challenge presented by the success of critical thought experiments.