
As quoted in His Brother's Blood: Speeches and Writings, 1838–64 https://books.google.com/books?id=qMEv8DNXVbIC&pg=PA193&lpg=PA193&dq=%22The+principle+of+enslaving+human+beings+because+they+are+inferior%22&source=bl&ots=YA6W9JoaPr&sig=aO15r4OJEVD8bQUIjM34u42GjXg&hl=en&sa=X&ved=0ahUKEwiM9vuXwsrLAhWJeD4KHWvpAUcQ6AEIHjAB#v=onepage&q=%22The%20principle%20of%20enslaving%20human%20beings%20because%20they%20are%20inferior%22&f=false (2004), edited by William Frederick Moore and Jane Ann Moore, p. 193
1860s, Speech to the U.S. House of Representatives (April 1860)