  On several occasions, especially in (1927) and (1939a), Fleck uses the term “incommensurability” (niewspółmierność) of concepts or ideas. He does not develop this issue. Yet it is easy—by looking through the Kuhnian glasses—to extract from his writings a quite well developed theory of incommensurability of thought styles. Its core is constituted by the claim that in the historical development of science there are no invariants.