Abe Lincoln is referring to Harriet Beecher Stowe, author of 'Uncle Tom's Cabin'. This book caused a change in the way Northerners thought about slavery, making them see the true evilness of it. However, the South didn't take to this book too kindly. They rang out protests on how it was "unfair" because Stowe herself had never witnessed slavery in the Deep South (where slavery was reportedly more familial and therefore better for the slaves). This book caused increasingly heated tensions between the North and the South, until it would hit the tipping point, the Civil War. In reference to Lincoln's quote, it is quite true that Harriet Beecher Stowe was at least one of the many causes of the Civil War.
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