We can distinguish three types of incommensurability in Kuhn’s remarks: (1) methodological—there is no common measure because the methods of comparison and evaluation change; (2) perceptual/observational—observational evidence cannot provide a common basis for theory comparison, since perceptual experience is theory-dependent; (3) semantic—the fact that the languages of theories from different periods of normal science may not be inter-translatable presents an obstacle to the comparison of those theories. (See Sankey 1993 for a useful discussion of Kuhn’s changing accounts of incommensurability.)