 Finally, further work in the philosophy of biology brings the level of the unit of selection down even further than the original genic selectionists do (Rosenberg 2006). Higher level selection is reducible to more fundamental  levels.[15]  Taking a reductionist stance, which is taken to be necessary to avoid an “untenable dualism” in biology between physicalism and antireductionism, the argument is that the principle of natural selection (PNS) should be properly viewed as a basic law of physical science (specifically chemistry), which can operate at the level of atoms and molecules (Rosenberg 2006: 189–191). Different molecular environments would favor different chemical types, and those that “more closely approximate an environmentally optimal combination of stability and replication,” are thus the “fittest” and would predominate (2006: 190). This could then be applied at each step of the way from simple molecules to compounds, organelles, cells, tissues, and so on, such that 