It is the aim of my contribution to analyze the relevance of nonrational elements in the history of science, more exactly the dialectics of scientific and unscientific roots of modern science. As a prerequisite I shall discuss, first of all, some general problems concerned with the rational reconstruction of the origin of modern science in respect to its unscientific or even counterscientific roots. Against the one-sidedness of two opposing views, namely "that science is fundamentally an irrational enterprise", and — in contradiction to it — that 'scientific behavior ... is the best example we have of rationality",¹ I want to stress the interplay of rational and irrational motives and ideas which is, in my view, vital for the advancement of scientific knowledge. By focussing attention on unscientific aspects in the actual history of science and by emphasizing the historical nature of scientific rationality, I do not share the relativism of Th. S. Kuhn and P. K. Feyerabend connected with the assumed incommensurability of theories and the rigid dichotomy of the cumulative and non-cumulative process in the development of science.² I am, however, fully aware that some of my views are controversial and will deserve a deeper justification from the philosophical point of view and by case studies as well. What I can offer here is only a brief outline of some substantial topics that support the basic idea of my approach, namely, that even outdated, metaphysical or counterscientific doctrines, religions, magical and mystical beliefs and opinions have positively influenced the rise and development of scientific knowledge.