In these essays, the author attacks the central problems in the philosophy of science from the viewpoint of a working scientist who relates the philosophical questions to methodological issues that have arisen in his own research. The essays deal with six main topics: the validation of hypotheses, the logic of causal ordering and its relation to the definability and identifiability of theoretical terms, the logic of imperatives and decisions, the structure of complex systems, the theory of scientific discovery, and the axiomization of scientific theories. He treats these topics in a unified way by employing the analytic tools of model theory and by putting the discovery of theories, rather than their testing, at the center of the stage. Questions surrounding the axiomization of theories are examined in the light of real examples (Newtonian mechanics and Ohm's law) rather than simplified, hypothetical ones. Formalisms are not avoided, but they are always used as a means and not as ends in themselves.--