A second style of thought that has contributed to existing theories of race is Marxism. Whereas American sociology's style of thought on race has consistently found race to exist in reality, the style of thought associated with historical materialism has refused to conceptualize race as a thing of reality. Moreover, unlike sociological race relations theory, the Marxist tradition has bequeathed to sociology (and social thought in general) a robust and parsimonious, if not controversial, theory of social knowledge. Insofar as Marxism has contributed to a theory of race, the account it presents is, on the surface, straightforward. Based on a radical rejection of biological explanations of race, Marxists of various stripes have treated race as a form of ideology. Thus Marxist thought offers a direct answer to the paradox of race, its simultaneous reality and unreality: race is not real.