Antithesis

antithesis: a balancing of two opposite or contrasting words, phrases, or clauses

"To be the friend of the one, is of necessity to be the enemy of the other."
                                            -Narrative of the Life of Frederick Douglass
                                                         by Frederick Douglass

Douglass' use of antithesis here is mainly to help with flow. Much of Douglass' book is composed of speeches, so smooth, flowing prose is a necessity. A halting, broken, or confusing speech will be quickly tuned out by the audience. Douglass made sure the quality of the speech was not the reason people did not listen, through the use of rhetorical devices such as antithesis. Douglass wanted to inspire the people to action against slavery, and to do this, made sure his speech was balanced and easy to listen to. Balancing the opposites also creates interesting phrasing and catches attention.