How did Frederick Douglass's life and accomplishments contradict arguments used to justify slavery in his time?

A chief way
whites justified slavery in the nineteenth century was through distorting and exaggerating
differences between white and black people. Black people were deemed innately intellectually and
morally inferior to whites. The writings of earlier influential philosophers, such as David
Hume, supporting false ideas that people of African descent were inferior, continued to have
strong currency in Douglass's lifetime. Hume, for example, wrote in his Essays, Moral,
Political and Literary,

I am apt to suspect the
negroes to be naturally inferior to the whites. There scarcely ever was a civilized nation of
that complexion, nor even any individual eminent either in action or speculation.

Philosopher Immanuel Kant designated the "Negro" as
"lazy, soft, and trifling." Even a writer who was highly sympathetic to black people,
Harriet Beecher Stowe, had so internalized racist stereotypes that she referred to them in
Uncle Tom's Cabin as "childlike."

There can
be no doubt there was a widespread...