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The pioneering study of W.E.B. Du Bois, The Philadelphia Negro: A Social Study (1899), inaugurated the sociological analysis of race issues. In the United States, when he began his investigation, the inequality in the living conditions amongst racial groups was commonly thought of as the consequence of the disparities between their respective and supposed biological characteristics. In a methodological and analytical tour de force, Du Bois disproved this widespread claim by articulating the then key concept of racial prejudice to that of social environment. By doing so, he defined racial prejudice not as a natural innate instinct but as a socially constructed attitude. This new perspective, both psychological and sociological, allowed Du Bois to outline the foundations of a relational theory of the social world. From this standpoint, his conclusions remain crucial for the social sciences, most notably for sociologists and social psychologists, often isolated within strictly delimited disciplinary boundaries.