 The views on scientific explanation have evolved away from the formal and cognitive accounts of the epistemic categories. Accordingly, the source of understanding provided by scientific explanations has been misidentified according to some (Barnes 1992). The genuine source for important, but not all, cases lies, in causal explanation, or causal mechanism (Cartwright 1983; Cartwright 1989; see also Glennan 1996, Cat 2005 and Craver 2007). Mechanistic models of explanation have become entrenched in philosophical accounts of the life sciences (Darden 2006, Craven 2007). As an epistemic virtue, the role of unification has been traced to the causal form of the explanation, for instance, in statistical regularities (Schurz 2015). The challenge extends to the alleged extensional link between explanation on the one hand, and truth and universality on the other (Cartwright 1983, Dupré 1993, Woodward 2003). In this sense, explanatory unity, which rests on metaphysical assumptions about components and their properties, also involves a form of ontological or metaphysical unity (for a methodological criticism of external, metaphysical perspectives, see Ruphy 2016).