  Some criticisms of the Unnaturalness Argument focus on the difficulties of defining “species.” As Jason Robert and Françoise Baylis (2003) note, although species can be defined in terms of reproductive isolation, genetic isolation, shared ancestry, or homeostatic property clusters, none of these definitions takes precedence over the others. Rather, “Biologists typically make do with a plurality of species concepts, invoking one or the other depending on the particular explanatory or investigative context” (Robert and Baylis 2003, 3).