What is the positive good theory?

The positive good theory is the idea that slavery was not, actually a "necessary evil," as Jefferson would describe it, but "a good-a positive good" institution for both blacks and whites in that whites get cheap manual labor and blacks benefit from the civilizing effect of being under the guidance of benevolent whites, and exposure to Christianity. The Positive good theory was John C. Calhoun's response during the Missouri crisis as to why slavery was continuing in the south. This theory became southern slaveholders justification from the 1820s through the Civil War. This idea, in similar form, was extended into the Jim Crow era, and is best represented in the film Birth of A Nation.