Fuller's Inner Morality of Law

In The Morality of Law (1964), Lon Fuller developed his reply to Hart with a parable about an imaginary ruler, King Rex, who fails to make law in eight different ways. Rex issues no general rules, keeps rules secret, makes them retroactive, writes them unclearly, makes them contradict each other, demands the impossible, changes them constantly, and acts in ways that do not match his own announced rules.

From these failures Fuller derived eight principles of legality: generality, promulgation, non-retroactivity, clarity, non-contradiction, possibility of compliance, constancy and congruence between official action and declared rules. He called these the 'inner morality of law', because a system that ignores them entirely fails to be law at all. Critics, including Hart, objected that these are principles of efficiency rather than morality, since even a wicked regime could follow them.

Fuller replied that respecting them treats people as responsible agents who can plan their lives, which is itself a moral commitment.