When theoretical statements use terms that refer to objects and properties whose existence seems awkward, puzzling, redundant, or ontologically problematic, there is motivation to analyze or reduce such statements to others that employ better understood terms. Reductionism must be distinguished both from eliminativism and supervenience. Consider two domains of properties M and P (e.g., the mental and the physical). Eliminativism claims that since only P exists, M can be eliminated (e.g., there is no such thing as demonic possession, but only a biochemical problem in the brain). Supervenience asserts that both M and P are real and distinct, though M is determined by P (e.g., headache pain is real, and while not identical to neurophysiological processes, is nonetheless realized by such processes). Reduction, however, asserts that there is but one thing that is...