My 'discovery' of the Polish School of philosophy of medicine stemmed from my studies in the genesis of Ludwik Fleck's epistemology. These studies, and my interest in the scientific roots of Fleck's epistemology were a nearly 'natural' result of my own biography: like Fleck I had been trained, an had worked as an immunologist, and had later switched to studies in the social history of medicine and biology. Moreover, it so happened that Fleck's book, Genesis and Development of a Scientific Fact -the description of a science as it is, not as it should be -was the first epistemological study in which I found echos of my experience in the laboratory. My interest in Fleck was also highlightened by the fact that in his works, and, as I discovered later, in the works of his predecessors of the Polish School of philosophy of medicine, was formulated the problem that had stimulated my interest in the history of medicine and biology, and is still central to my present investigations: the relationships between biological knowledge and clinical practice. The writing of the book was made possible through to the help of many colleagues and friends. The unfailing support for my research, whatever its subject might be, from my colleagues from Unit 158 of INSERM and in particular from its head Patrice Pinell, has made my study of the Polish School possible.