 Reduction (reductionism) encompasses several, related philosophical themes. At least three types can be distinguished: ontological, methodological, and epistemic (Sarkar 1992; cf. Nagel 1998). Even though arguments for and against reductionism often include a combination of positions related to all three, these distinctions are significant because no straightforward entailment relations obtain between the different types of reduction (although tacit commitments about these relations are prevalent).