The paper discusses the main themes of the Kuhn Popper controversy. Popper defends the possibility of a rational reconstruction of science, objective truth and normative methodology. Kuhn emphasizes the historical and sociological aspects of the development of science, the relativity of truth, and the incommensurability of paradigms. The author confronts these two opposing conceptions of the philosophy of science and applies them to the historical case of Ignaz Semmelweis who proposed a revolutionary theory of childbed fever, anticipating the germ theory officially ascribed to Pasteur and Lister. Kuhn’s view on the resistance of scientific communities to a paradigm threat may explain why Semmelweis’ discovery was rejected. However, the sociological and psychological explanation is accompanied by the loss of the normative function of the philosophy of science. In opposition to Kuhn, Popper formulates a critical imperative as the rational core of science and the guarantee of open, creative thinking. According to Popper, the philosophy of science must not only explain Semmelweis’ failure with external reasons, but must also prescribe the methodological norms that eliminate dogmatism from rationality.