Why does Paton depend on opposites to present his ideas ?

I think the apartheid era of South Africa clearly represented opposites in many ways. Blacks and whites were segregated socially, emotionally, and economically. Apartheid was designed to illuminate the opposites so that white society could remain dominant.  The capital city of Johannesburg seems to illustrate this point. It is a city where blacks, like Absalom and Gertrude, are marginalized