There is wide agreement that new scientific theories often provoke a protracted discussion of their epistemic merits and their metaphysical presuppositions. Action-at-a-distance, vital forces, evolutionary theory, and the atomic debates are familiar cases in point. It has not, however, been widely appreciated that subtle fluid theories during their hey-day in the 18th and 19th centuries produced a philosophical discussion which was at least as deep as, and probably more far-reaching than, those associated with atomism and non-contact action. The aim of this chapter is to explore briefly some of the philosophical aspects of "the ether debates", in order to document the impact of those debates on the fortunes of subtle fluid physics and on the 19th-century revision of the methodology of empiricism.