In this posthumous book, the late Professor R. Hooykaas (1906-1994) conveys a lifetime of historical thought about modes of scientific advance over the centuries. In what variety of ways has the human mind, with all its subjectivity and its capacity for self-deception, but also its piercing gifts of discovery, managed to come to terms with `the whimsical tricks of nature? Central to this erudite, penetrating, and widely ranging study is Hooykaass distinction between facts (given by nature yet entirely subject to our mode of interpreting them), faith (broad conceptions like the idea of order, of simplicity, or of harmony), and fictions in the sense of those daring intellectual tools, such as theories and hypotheses and models, which reflect the scientists creative imagination. Case studies drawn from the history of all branches of science (including chemistry and the earth sciences) and from Antiquity to the present day, serve to widen and to deepen the understanding of every reader (whether a historian of science or not) with a desire to learn more about the realities of the scientific pursuit.