Abraham Lincoln knew about the slaves through various means. He grew up in Kentucky, a state where slavery was legal, and would have had firsthand experience of the institution from a young age. He also met and spoke to many slaves throughout his life, including during his time as a lawyer when he represented enslaved individuals in court. Lincoln also read widely on the subject of slavery, including the writings of abolitionists and former slaves, which helped him to understand the plight of slaves. Additionally, he had many discussions with political and social leaders who were involved in the debate over slavery, which further deepened his knowledge and understanding of the issue.