 A weaker form of ontological reduction advocated for the biomedical sciences with the causal notion of partial reductions: explanations of localized scope (focused on parts of higher-level systems only) laying out a causal mechanism connecting different levels in the hierarchy of composition and organization (Schaffner 1993; Schaffner 2006; Scerri has similarly discussed degrees of reduction in Scerri 1994). An extensional, domain-relative approach introduces the distinction between “domain preserving” and “domain combining” reductions. Domain-preserving reductions are intra-level reductions and occur between \(T_1\) and its predecessor \(T_2\). In this parlance, however, \(T_2\) “reduces” to \(T_1\). This notion of “reduction” does not refer to any relation of explanation (Nickles 1973).