How did Frederick Douglass's life and accomplishments contradict arguments used to justify slavery in his time?
Contemporary justifications for slavery tended to take up two separate strands. The first was that blacks were inherently inferior to whites, particularly in their intellect. It was widely thought that they were incapable of abstract reasoning, and that their minds were best suited to the kinds of drudgery that characterized slavery in almost all of its forms. The second was that the institution of slavery was in fact a positive good, and slaves, inasmuch as they were deemed racially inferior, benefited from it. In fact, slaves, it was argued, were happy within the institution.
The career of Frederick Douglass gave the lie to both of these assumptions. Beginning with the first, he was an extraordinarily eloquent speaker, and a gifted writer. His speeches were full of classical allusions, references to the history of the United States and rhetorical flourishes that made him perhaps the most famous speaker of his day. Douglass was an essentially self-educated black man who violated racial constructs with every stroke of his pen and every speech he gave.
Nor was Douglass a "happy slave." In Narrative of the Life of Frederick Douglass, he described his life in slavery as brutal and degrading, and attributed any apparent intellectual inferiority on the part of slaves not to immutable racial characteristics, but to the utterly degrading circumstances of slavery. Under slavery, he wrote:
My natural elasticity was crushed, my intellect languished, the disposition to read departed, the cheerful spark that lingered about my eye died; the dark night of slavery closed in upon me; and behold a man transformed into a brute!
Douglass was the living embodiment of everything that proslavery writers claimed was impossible.