  (a) The language used to describe what to members of a collective appears as reality changes. Some words disappear, other appear, and even if words remain their meanings change to a greater or lesser extent. Fleck quotes some fragments of writings of a physicist James Clerk Maxwell and a philosopher Henri Bergson on motion and knowledge on motion and he concludes that the words “motion” and “knowledge” have different meanings for them: “it is not as if the word of one of them meant a thing that the other would give a different name, but that a thing to which one of them gave a certain name does not exist at all for the other one” (1936, II). We can find similar examples in writings of “scientists”. Heat in the second half of the 18th century was on the list of chemical elements, and in the 19th century it became the motion of atoms. What Lavoisier or Carnot called “heat”, for Maxwell or Boltzmann did not exist and vice versa. So if they could meet and talk they would not make inconsistent claims about the same phenomenon, but would speak of something different. “It is not possible to express with today’s words the content of a view of a distant epoch since particular notions of this epoch are incommensurable with the ones of today” (1939a).