This article analyses three basic concepts of Ludwik Fleck’s Genesis and Development of a Scientific Fact. It shows first that Fleck’s notion of „directed perception“ is closely linked to Jakob von Uexküll’s writings on the „Umwelt“ of animals and humans. The article then proposes to regard the epistemological debates surrounding parapsychology as an important testing ground for the Fleckian concept of „mood“ and his concomitant hypotheses about „the tenacity of systems of opinion and the harmony of illusions“. It finally argues (...) that Fleck’s modification of Wilhelm Jerusalem’s idea of the „social consolidation“ of knowledge helps us to understand the indebtedness of Fleck towards early functionalist sociology as well as his strong belief in „specific historical laws governing the development of ideas“. The historical semantics of Fleck’s works hence proves that his insights are neither marginal nor revolutionary but rather deeply rooted within scientific traditions from the late 19th and early 20th centuries. (shrink)