From this novel, we learn that part of the process of creating docile and obedient slaves, for many slave holders, is breaking down the slaves' sense of selfhood through the denial of their freedom. While Mr. Garner does not necessarily employ this method with his slaves while he is alive, the other slave owners he knows, as well as schoolteacher (who takes over Sweet Home after Mr. Garner's death) and Brandywine (the slaver that Paul D tries to kill after he was sold from Sweet Home), seem to do this.
Schoolteacher, for example, has his nephews comparing Sethe's "human" qualities and her "animal" qualities, dehumanizing and degrading her, showing how he thinks of slaves and, thus, how he treats them. Clearly his nephews learn to think of her as less-than-human as well because they assault her, stealing her milk, and whip her mercilessly, creating the web work of scars she has on her back.
Paul D is forced to wear an iron bit in his mouth and a spiked collar is put on him, like an animal, giving him lots of time to reflect on how even animals like the rooster actually have more freedom and selfhood than he does. Paul D says of the rooster,
Mister, he looked so...free. Better than me. Stronger, tougher ... Mister was allowed to be and stay what he was. But I wasn't allowed to be and stay what I was. Even if you cooked him you'd be cooking a rooster named Mister. But wasn't no way I'd ever be Paul D again, living or dead.
Slavery, essentially, turns human beings into animals, into something even less than animals, denying them the experience of selfhood to which all humans are entitled.