 Most discussion in contemporary philosophy of science has centered on the issue of  epistemic reduction  (rather than ontological or methodological reduction; see Hoyningen-Huene 1989). The basic idea that the knowledge (or other epistemic units) pertaining to one scientific domain can be reduced to another body of knowledge (or its epistemic units) has been spelled out in different ways. It is useful to group different approaches to reduction into two basic categories: (1) models of  theory reduction,  which maintain that a higher level theory can be logically deduced from a lower level theory, and, (2) models of  explanatory reduction,  which focus on whether representations of higher level features can be explained by representations of lower level features, typically by decomposing a higher level system into parts (Sarkar 1992).  Models of theory reduction  were of primary interest in post-positivist philosophy of science that emerged in the 1960s and have received the most sustained attention (Schaffner 1993). More recently, accounts of explanatory reduction have been developed to remedy the inadequacies of theory focused accounts, so that theory reduction has been displaced as the central issue by competing accounts of explanatory reduction (but see Dizadji-Bahmani et al. 2010).