“An authoritative command must … be distinguished from a persuasive argument. When I am commanded to do something, I may choose to comply even though I am not being threatened, because I am brought to believe that it is something which I ought to do. If that is the case, then I am not, strictly speaking, obeying a command, but rather acknowledging the force or rightness of a prescription. … But the person himself [sic] has no authority—or, to be more precise, my complying with his command does not constitute an acknowledgment on my part of any such authority.”