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Quote :The Souls of Black Folk
[T]he Negro is a sort of seventh son, born with a veil, and gifted with second-sight in this American world,—a world which yields him no true self-consciousness, but only lets him see himself through the revelation of the other world. It is a peculiar sensation, this double-consciousness, this sense of always looking at one's self through the eyes of others, of measuring one's soul by the tape of a world that looks on in amused contempt and pity. One ever feels his two-ness,—an American, a Negro; two souls, two thoughts, two unreconciled strivings; two warring ideals in one dark body, whose dogged strength alone keeps it from being torn asunder.
If you're black, you can't just be yourself in America.
That's because you're a minority in a white majority culture that looks down on you. When you look at yourself, you see yourself through the eyes of this white majority culture. You see yourself with all the negative stereotypes that white America attaches to you. You're stuck in this "double-consciousness" because you're forced to see yourself not just as a person, but as a black person.
Du Bois's discussion of "double-consciousness" represents one of the first attempts on the part of a person of color to theorize what it means to be an ethnic or racial minority in white America. There's a reason his book became a big influence on Ethnic Studies, 70 years after it was written.