Jubal groaned. "Duke, how could you learn so much about machinery and never learn anything about how you yourself tick?  That nausea you feel - that's not an instinct; that's a conditioned reflex.  Your mother didn't have to say to you, 'Mustn't eat your playmates, dear; that's not nice,' because you soaked it up from our whole culture - and so did I.  Jokes about cannibals and missionaries, cartoons, fairy tales, horror stories, endless little things.  But it has nothing to do with instinct.  Shucks, son, it couldn't possibly be instinct... because cannibalism is historically one of the most widespread of human customs, extending through every branch of the human race.  Your ancestors, my ancestors, everybody."