 Although the theory-dependence of observation plays a significant role in The Structure of Scientific Revolutions, neither it nor methodological incommensurability could account for all the phenomena that Kuhn wanted to capture with the notion of incommensurability. Some of his own examples are rather stretched—for instance he says Lavoisier saw oxygen where Priestley saw dephlogisticated air, describing this as a ‘transformation of vision’ (1962/1970a, 118). Moreover observation—if conceived of as a form of perception—does not play a significant part in every science. Kuhn wanted to explain his own experience of reading Aristotle, which first left him with the impression that Aristotle was an inexplicably poor scientist (Kuhn 1987). But careful study led to a change in his understanding that allowed him to see that Aristotle was indeed an excellent scientist. This could not simply be a matter of literally perceiving things differently. Kuhn took the incommensurability that prevented him from properly understanding Aristotle to be at least partly a linguistic, semantic matter. Indeed, Kuhn spent much of his career after The Structure of Scientific Revolutions attempting to articulate a semantic conception of incommensurability.