 The claim that reduction, as a relation of explanation, needs to be a relation between theories or even involve any theory has also been challenged. One such challenge focuses on “inter-level” explanations in the form of compositional redescription and causal mechanisms (Wimsatt 1976). The role of biconditionals or even Schaffner-type identities, as factual relations, is heuristic (Wimsatt 1976). The heuristic value extends to the preservation of the higher-level, reduced concepts, especially for cognitive and pragmatic reasons, including reasons of empirical evidence. This amounts to rejecting the structural, formal approach to unity and reductionism favored by the logical-positivist tradition.  Reductionism is another example of the functional, purposive nature of scientific practice. The metaphysical view that follows is a pragmatic and non-eliminative realism (Wimsatt 2006). As a heuristic, this kind of non-eliminative pragmatic reductionism is a complex stance. It is, across levels, integrative and intransitive, compositional, mechanistic and functionally localized, approximative and abstractive. It is bound to adopting false idealizations, focusing on regularities and stable common behavior, circumstances and properties. It is also constrained in its rational calculations and methods, tool-binding, and problem-relative.  The heuristic value of eliminative inter-level reductions has been defended as well (Poirier 2006).