When that leftie friend starts yammering about Slavery, show him what Ishmael said:

“What of it, if some old hunks of a sea-captain orders me to get a broom and sweep down the decks? What does that indignity amount to, weighed, I mean, in the scales of the New Testament? Do you think the archangel Gabriel thinks anything the less of me, because I promptly and respectfully obey that old hunks in that particular instance? Who ain’t a slave? Tell me that. Well, then, however the old sea-captains may order me about—however they may thump and punch me about, I have the satisfaction of knowing that it is all right; that everybody else is one way or other served in much the same way— either in a physical or metaphysical point of view, that is; and so the universal thump is passed round, and all hands should rub each other’s shoulder-blades, and be content.”

— Moby Dick, Chapter 1, page 16.