President Abraham Lincoln very succinctly explained his rationale for his condemnation of and opposition to slavery, to wit: "As I would not be a slave, so I shall not be a master." Lincoln's reasoning best exemplifies what ethical theory?

President Abraham Lincoln very succinctly explained his rationale for his condemnation of and opposition to slavery, to wit: "As I would not be a slave, so I shall not be a master." Lincoln's reasoning best exemplifies what ethical theory?