 In a general semantic account, Sarkar distinguishes different kinds of reduction in terms of four criteria, two epistemological and two ontological: fundamentalism, approximation, abstract hierarchy and spatial hierarchy. Fundamentalism implies that the features of a system can be explained in terms only of factors and rules from another realm. Abstract hierarchy is the assumption that the representation of a system involves a hierarchy of levels of organization with the explanatory factors being located at the lower levels. Spatial hierarchy is a special case of abstract hierarchy in which the criterion of hierarchical relation is a spatial part-whole or containment relation. Strong reduction satisfies the three “substantive” criteria, whereas weak reduction only satisfies fundamentalism. Approximate reductions—strong and hierarchical—are those which satisfy the criterion of fundamentalism only approximately (Sarkar 1998; the merit of Sarkar’s proposal resides in its systematic attention to hierarchical conditions and, more originally, to different conditions of approximation; see also Ramsey 1995; Lange 1995; Cat 2005).