Since its almost serendipitous rediscovery in the late seventies, Fleck's monograph, Entstehung und Entwicklung einer wissenschaftlichen Tatsachee, initially published in 1935, translated into English in 1979 (Genesis and Development of a Scientific Fact), has been met with increasing acclaim within the philosophy and the sociology of science. In historizing, sociologizing and relativizing science, Fleck is claimed to have expressed prescient views on the history, philosophy and sociology of science and in deeply influencing Kuhn. Though the neglect of Fleck by his contemporaries has been difficult to account for, the basis of his epistemology has evoked little interest, partly due to the lack of apparent sources. Fleck's philosophical writings, published between 1927 and 1939, indicate, however, a polemic, deeply ingrained in an ongoing debate, on the standing of old established scientific disciplines versus new and emerging ones, occasioned by the rapid changes within the natural sciences. Most obvious to the lay community, and also reflected in the new positivist philosophy, were the revolutionary changes within physics. As a participant in the debate, eagerly striving for recognition, Fleck used modern physic heuristically as the basis of his epistemology. The tracing of his sources, and the voices of other contemporaneous scientists opposing his views, are attempted.