I examine some logical and epistemological problems arising in quantum theory. Two historical episodes are considered in detail: Planck's adoption of the quantum hypothesis and Heisenberg's discovery of matrix mechanics. The former episode is, I argue, more compa tLbLe with logical empiricist views of scientific progress than it is with more recent and currently influential philosophies. My examination of the latter concerns the influences - epistemological as well as.sci~ntific- which may have affected the manner in which Heisenberg chose to present. his discoveries; these considerations a~low me to rebut ... the suggestion that Heisenberg's practice was incompatible with his professed views and to counter the claim that this episode provided unwarranted support to formalistic ideas concerning scientific explanation. The connexions between Bohr's principle of complementarity and logical empiricist views concerning language change in science are explored, and I argue that the transition from classical to quantum mechanics· can be seen as involving a shift in the meanings of "position" and "momentum" so that these terms obtain unambiguous empirical meanings from the viewpoint of quantum· theory. Discussing the relevance of quantum mechanics to logic I contend that, although classical logic may be retained, empirical considerations may make it advisable to adopt a non-classical logic for quantum theory. An analysis of Bell' s inequality.:is a prelude to some speculations concerning the interpretation of quantum theory. I examine the possibility of interpreting the formalism as descriptive not of an external world but of the expectati.ons of subjects. The final chapter explores the relevance of physics to questions concerning human freedom. Classical physics, I argue, did not have the devastating implications for personal autonomy that it was often assumed to have; quantum physics does not explain . human freedom but, under certain assumptions, it does show how it is possible.