  For the most part, philosophical criticism of Appiah’s reading of Du Bois has targeted his analysis of Du Bois’s definition and his antirealism about race. In an early response to Appiah, Lucius Outlaw (1996, 28) argues that Du Bois’s definition of race is the articulation of a cluster concept, and not, as Appiah presumes, the statement of a set of conditions individually necessary and jointly sufficient to constitute a group of people as a  race.[13]  In addition, Outlaw (1996, 21–22, 28) defends, and attributes to Du Bois, a version of racial realism, according to which biological ancestry and physical characteristics play a role in defining race.