I'm reading the Demon Haunted World by Carl Sagan
In it he quotes Fredrick Baily a slave from Maryland in the 1820's. Baily wrote after escaping to Maine these words :
To make a slave contented it was necessary to make a thoughtless one. It is necessary to darken his moral and mental vision, and as far as possible, to annihilate the power of reason. This is why slaveholders control what slaves hear, see and think. This is why reading and critical thinking are dangerous, indeed subversive in an unjust society.
If you havn't read the book, it's excellent.