Stamped from the Beginning: The Definitive History of Racist Ideas in America by Ibram X. Kendi:
Equating enslaved Blacks to three-fifths of all other (White) persons matched the ideology of racists on both sides of the aisle. Both assimilationists and segregationists argued, yet with different premises and conclusions, that Black people were simultaneously human and subhuman. Assimilationists stridently declared the capability of sub-White, sub-human Blacks to become whole, five-fifths, White, one day. For segregationists, three-fifths offered a mathematical approximation of inherent and permanent Black inferiority. They may have disagreed on the rationale and the question of permanence, but seemingly all embraced Black inferiority—and in the process enshrined the power of slaveholders and racist ideas in the nation’s founding document.