Legitimacy of authority and the duty to obey

What's the relationship between the moral legitimacy of an authority and one's duty of fidelity to it?
Greenawalt: The legitimacy of a political authority does not define the scope of one's duty to obey that authority. An illegitimate authority can issue directives that carry an obligation to obey, and a legitimate authority can issue directives that do not. Possible substantive grounds for the duty to obey include promises, utility, fair play and a natural duty.

How does Fuller account for the duty to obey?
Distinguishes b/t the morality of aspiration ("the good life" a la the Greeks) and the morality of duty ("basic rules without which an ordered society is impossible, or without which an ordered society directed toward certain specific goals must fail of its mark"--the Old Testamant and the 10 Commandments). Morality of aspiration can have no direct bearing on the requirements of law; for workable standards, law must adhere to the morality of duty...(to be continued)

How can a positivist?