Is morality just something we invent, so that we could invent a different “morality” if we wanted to?  Is it something like an instinct, so that we might have developed with a different “morality,” like fish who eat their young?  Is it the capricious command of a God who might commanded just the opposite, like “Thou shalt murder your friends?”  All of these answers make morality arbitrary — and all of these answers are wrong.