 Racial skepticism holds that because racial naturalism is false, races of any type do not exist. Racial skeptics, such as Anthony Appiah (1995, 1996) and Naomi Zack (1993, 2002) contend that the term race cannot refer to anything real in the world, since the one thing in the world to which the term could uniquely refer—discrete, essentialist, biological races—have been proven not to exist. Zack (2002, 87–88) provides an accessible summary of the racial skeptic’s argument against the biological foundations for race, sequentially summarizing the scientific rejection of essences, geography, phenotypes, post-Mendelian transmission genetics, and genealogies as possible foundations for races. Aristotelian essences, thought to ground the common characteristics of distinct species, were correctly rejected by early modern philosophers. If essences cannot even ground differences among species, then they clearly cannot ground the differences among races, which even nineteenth century racial science still understood as members of the same species. Whereas folk theories rely on geography to divide humanity into African, European, Asian, and Amerindian races, contemporary population genetics reveal the vacuity of this foundation for two reasons. First, geographically based environmental stimuli lead to continuous physical adaptations in skin, hair and bone rather than the discrete differences associated with race; and second, although mitochondrial DNA mutations provide evidence of the geographical origins of populations, these mutations do not correlate with the physical traits associated with racial groups. Similarly, phenotypes cannot ground folk theories of race: for instance, differences in skin tone are gradual, not discrete; and blood-type variations occur independently of the more visible phenotypes associated with race, such as skin color and hair texture. Race cannot be founded upon transmission genetics, since the genes transmitted from one generation to the next lead to very specific physical traits, not general racial characteristics shared by all members of a putatively racial group. And finally, genealogy cannot ground race, since clades (populations descended from a common ancestor) may have common genetic characteristics, but these need not correlate with the visible traits associated with races. Zack concludes: “Essences, geography, phenotypes, genotypes, and genealogy are the only known candidates for physical scientific bases of race. Each fails. Therefore, there is no physical scientific basis for the social racial taxonomy” (Zack 2002, 88).