There was a most revealing rule: slaves were to remain illiterate. In the antebellum South, whites who taught a slave to read were severely punished. '[To] make a contented slave,' Bailey later wrote, 'it is 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 the slaveholders must control what slaves hear and see and think. This is why reading and critical thinking are dangerous, indeed subversive, in an unjust society.
Source: The Demon-Haunted World
by Carl Sagan