 Reductionism encompasses a set of ontological, epistemological, and methodological claims about the relations between different scientific domains. The basic question of reduction is whether the properties, concepts, explanations, or methods from one scientific domain (typically at higher levels of organization) can be deduced from or explained by the properties, concepts, explanations, or methods from another domain of science (typically at lower levels of organization). Reduction is germane to a variety of issues in philosophy of science, including the structure of scientific theories, the relations between scientific disciplines, the nature of explanation, the diversity of methodology, and notions of theoretical progress, as well as to numerous topics in metaphysics and philosophy of mind, such as emergence, mereology, and supervenience.