Why is it great? Take a look at the publication date: 1792. That's more than two centuries ago, and two things are remarkable about this fact. 1) That Wollstonecraft, the mother of "Frankenstein" author Mary Shelley, was such a terrific feminist back then. And 2) that in many respects it still holds true. But the sentence is a lyrical wonder, creating an image that brings to mind "I Know Why the Caged Bird Sings." (In reading up on Wollstonecraft, I learned that she died soon after giving birth to Mary. And her widower told a friend: "I firmly believe there does not exist her equal in the world. I know from experience we were formed to make each other happy. I have not the least expectation that I can now ever know happiness again.")