For Habermas, contrary to what the legal positivists and system theorists believe, law requires legitimation (and thus is dependent on the discourse of morality) in post-traditional societies too, even if no natural-law metaphysics satisfies that desideratum. Habermas is aware that neither the appeal to a classical "philosophy of subjectivity" nor a notion of "de-limited communication" is capable of supplying the necessary legitimation. In his book Faktizität und Geltung ¹, it is this basic dilemma that constitutes the problem to which a "discourse theory of law" seeks the answer.