The Spirits' Book
Book Three - Moral Laws
Chapter X - IX. Law of Liberty
829. Is there any portion of the human race intended by nature to be the property of other human beings?
“The absolute oppression of any individual by another violates God’s law. Slavery is an abuse of strength and gradually disappears with progress, as all other abuses will eventually disappear.”
The societal law that sanctions slavery is a crime against nature, because it reduces human beings to the level of animals, and degrades them both physically and morally.