  What Fleck calls active elements of a thought style, Henri Poincaré calls “principles” and sees as “disguised definitions”. Poincaré stresses that there is nothing necessary in definitions—they can be various and they are chosen for pragmatic and aesthetic reasons. Fleck agrees with him to a certain extent. He notices that in the 19th century syphilis was defined in many ways: as a carnal scourge, as a mercury-treated decease, and finally as a set of typical symptoms. And he claims that “(…) if syphilis can be defined in various ways, the definition selected still determines some conclusions. (…) there are some associations which are open to choice (…) and others that are constrained” (1935a, I). For example if one defined syphilis as a “carnal scourge”, then other diseases like gonorrhea, soft chancre, etc. would be cases of syphilis; and when proper experiments had been conducted one stated for example that “sometimes mercury does not cure the carnal scourge but makes it even worse”. Looking at the same experiment those who defined syphilis as a disease which can be treated with mercury would conclude that they are not dealing with syphilis. However, Fleck opposes the conventionalist idea that definitions can be chosen freely. Definitions are born with the birth of a thought style, and their formulation is determined by both historical-cultural conditioning and by the process of exchanging ideas among researchers which was described above. A mature thought style is a system where definitions mutually condition each other and where accepting a certain definition can prevent accepting some other ones.