Why does grimke call the abolitionist movement the nations foremost school of human rights?

Grimké was an American political activist and supporter of the women's suffrage movement. Her series of twelve letters forthrightly defending the right of woman to take part in political debate. The final one addressed the question of human rights directly. She also declares that whatever is morally right for man to do is morally right for woman to do. That is why, Angelina Grimke call the abolitionist movement the nation’s foremost "school of human rights".