 Of all the sources influencing Kuhn on incommensurability, at least one deserves special attention. In the foreword of The Structure of Scientific Revolutions, Kuhn acknowledged a deep debt to Ludwik Fleck, a bacteriologist who developed the first explicit sociology of natural science, and anticipated many contemporary views about the social construction of knowledge. Around 1950, Kuhn was enticed by the potential relevance to his experience of incommensurability of Fleck’s paradoxical title: Entstehung und Entwicklung einer wissenschaftlichen Tatsache: Einführung in die Lehre von Denkstil und Denkkollektiv (1935)(Genesis and Development of a Scientific Fact, 1979). There and in other earlier works, Fleck had already used ‘inkommensurabel’ to describe different styles of thinking within the natural sciences as well as to discuss the ramifications of radical conceptual change in the history of science. For example, Fleck used the term ‘inkommensurabel’ to describe the differences between ‘medical thinking’ and ‘scientific thinking’. The former addresses irregular, temporally dynamic phenomena such as an illness, while the later addresses uniform phenomena (Fleck 1986 [1927], 44–45). Fleck also used the term ‘inkommensurabel’ to describe conceptual replacements in theoretical transitions within what he considered the most vital of the natural sciences, the medical sciences. For example, he claimed that an old concept of disease became incommensurable with a newer concept that was not a completely adequate substitute for it (Fleck 1979 [1935], 62). While Fleck’s program of comparative epistemology anticipates Kuhn’s ideas in many significant respects, it is also strikingly different (Harwood 1986; Oberheim 2005; Peine 2011). The most pervasive differences concerning incommensurability are that Fleck treats meaning and meaning change as a function of how concepts are received and developed by the collective, while for Kuhn it is individuals who develop and apply the concepts that advance science. Moreover, for Fleck meaning change is a continuous feature of scientific development, whereas Kuhn distinguishes normal scientific development that does not change meanings from revolutionary developments that do.