A less stated (but well understood) component of this story is that Clemens grew up believing in his southern slave culture, fought (if joining up is fighting) in defence of the southern side, but converted later in life, not the least because he married into an abolitionist family.
A quote I like: "Samuel Clemens later observed, "Civilization began when slavery was abolished."
Many in the south remained unreconstructed racists and opposed to Grant. Clemens didn't.
I suppose it depends on your definition of civilisation. I agree that emotionally the quote conveys a laudable sentiment but technically, it would probably be more correct to state the opposite i.e. that civilisation began with slavery. As the beginnings of civilisation are usually correlated with the the beginnings of agriculture and agriculture, arguably, created the demand for workers which allowed slavers to flourish.
A quote I like: "Samuel Clemens later observed, "Civilization began when slavery was abolished."
Many in the south remained unreconstructed racists and opposed to Grant. Clemens didn't.