If there is no struggle, there is no progress. Those who profess to favor freedom, and yet deprecate agitation, are men who want crops without plowing up the ground, they want rain without thunder and lightning. They want the ocean without the awful roar of its many waters. This struggle may be a moral one; or it may be a physical one; or it may be both moral and physical, but it must be a struggle. Power concedes nothing without a demand. It never did, and it never will. Find out just what people will submit to, and you have found out the exact amount of injustice and wrong which will be imposed upon them; and these will continue until they are resisted with either words or blows, or both. The limits of tyrants are proscribed by the endurance of those whom they oppress... People might not get all that they work for in this world, but they must certainly work for all they get.

[Note: I originally only had the 'limits of tyrants' part quoted here. It's a good phrase, very empowering, entirely quotable, and points the finger right at the problem. But I think the quote, as a whole, is even better, and yet likely still falls short of expressing all the great things this man had to say.]