"There is great sincerity and simplicity in the writings which record the sayings of Epictetus ... His morality is lofty and unworldly; in a situation in which man's main duty is to resist tyrannical power, it would be difficult to find anything more helpful. In some respects, for instance in recognizing the brotherhood of man and in teaching the equality of slaves, it is superior to anything to be found in Plato or Aristotle or any philosopher whose thought is inspired by the City State. The actual world, in the time of Epictetus, was very inferior to the Athens of Pericles; but the evil in what existed liberated his aspirations, and his ideal world is as superior to that of Plato as his actual world is inferior to the Athens of the fifth century." - Bertrand Russell, The History of Western Philosophy
For more on Epictetus, see his entry at the Internet Encyclopedia of Philosophy.