 Other models of explanatory reduction have been put forward by philosophers, many with an explicit eye to capturing how reduction occurs in scientific practice (e.g., Bickle 2003, 2006, 2008). In some ways the most unique defense of explanatory reduction is found in the recent work of Alex Rosenberg (2006), which departs from his earlier critical focus on theory reduction (1978, 1985, 1994). First, strict laws (universal, exceptionless, spatio-temporally unrestricted) are required for explanation and the only candidate law in biology is the principle of natural selection (PNS). Second, why-necessary explanations are better than how-possible explanations in historical sciences such as biology, but why-necessary explanations are only available at the molecular level because structure becomes decoupled from function above this level. Therefore, all how-possible explanations in “functional biology” (i.e., non-molecular biology), even those invoking the PNS, and any descriptions from functional biology involving higher levels of organization get explained (why-necessarily) by the PNS operating on the molecular level (often occurring at some relatively distant point in evolutionary  history).[6]