Frederick Douglass 

Douglass, a former slave, was the most eloquent abolitionist; he wrote a best-selling autobiography, traveled the north to give speeches, published the North Star newspaper and (later, during the Civil War) encouraged Lincoln to free the slaves and helped to organize black regiments for the Union Army

In his time he was described by abolitionists as a living counter-example to slaveholders' arguments that slaves lacked the intellectual capacity to function as independent American citizens. Northerners at the time found it hard to believe that such a great orator had once been a slave.

The "Mud-Sill" Theory
Excerpt from Senator James Hammond (South Carolina) speech to the U.S. Senate, March 4, 1858

In all social systems there must be a class to do the menial duties, to perform the drudgery of life. That is, a class requiring but a low order of intellect and but little skill. Its requisites are vigor, docility, fidelity. Such a class you must have, or you would not have that other class which leads progress, civilization, and refinement. It constitutes the very mud-sill [or foundation] of society and of political government; and you might as well attempt to build a house in the air, as to build either the one or the other [without] this mud-sill. Fortunately for the South, she found a race adapted to that purpose to her hand. A race inferior to her own, but eminently qualified in temper, in vigor, in docility, in capacity to stand the climate, to answer all her purposes. We use them for our purpose, and call them slaves.

We do not think that whites should be slaves either by law or necessity. Our slaves are black, of another and inferior race. The status in which we have placed them is an elevation. They are elevated from the condition in which God first created them by being made our slaves. None of that race on the whole face of the globe can be compared with the slaves of the South. They are happy, content, unaspiring, and utterly incapable, from intellectual weakness, ever to give us any trouble by their aspirations.

I was somewhat unmanageable when I first went there, but a few months of this discipline tamed me. Mr. Covey succeeded in breaking me. I was broken in body, soul, and spirit. 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!

 Sunday was my only leisure time. I spent this in a sort of beast-like stupor, between sleep and wake, under some large tree. At times I would rise up, a flash of energetic freedom would dart through my soul, accompanied with a faint beam of hope, that flickered for a moment, and then vanished. I sank down again, mourning over my wretched condition. I was sometimes prompted to take my life, and that of Covey, but was prevented by a combination of hope and fear. My sufferings on this plantation seem now like a dream rather than a stern reality. 

“Our house stood within a few rods of the Chesapeake Bay whose broad bosom was ever white with sails from every quarter of the habitable globe. Those beautiful vessels robed in purest white, so delightful to the eye of freemen were to me so many shrouded ghosts, to terrify and torment me with thoughts of my wretched condition. I have often, in the deep stillness of a summer Sabbath, stood all alone upon the lofty banks of that noble bay and traced, with saddened heart and tearful eye, the countless number of sails moving off to the mighty ocean. The sight of these always affected me powerfully. My thoughts would compel utterance, and there with no audience but the Almighty I would pour out my soul’s complaint in my rude way, with an apostrophe to the moving multitudes of ships. You are loose from your moorings and are free. I am fast in my chains and am a slave. You move merrily before the gentle gale and I sadly before the bloody whip."

Narrative of the Life of Frederick Douglass, An American Slave (Chapter X)