The correct answer is: constitution and the concept of slavery.
Although Lincoln was never a self-proclaimed abolitionist, he held a personal view that slavery is anti-human, but he also held a political view that it was an unconstitutional practice. Still, as the president of all U.S. citizens (many of whom were anti-abolitionists and slave owners), he had to be very careful. He never advocated the outright and sudden abolition of slavery, as he was aware of all the consequences. Instead, he advocated a gradual change.
