Abstract: A great variety of propositions of law are used in legal practice. Does a proposition of law have its true value? What does it mean to say that a proposition of law is true? In answering these questions, Patterson criticizes legal formalism of Ernest Weinrib, moral realism of Michael Moore, semantics theory of David Brink, legal positivism of Hart and Dworkin and law interpretation theory of Stanley Fish. Moreover, combining the “modalities” (forms of argument) that Philip Bobbitt raised in constitutional interpretation and W. V. Quine’s “maxim of minimum mutilation”, he puts forward his own theory of propositions of law and explains the possibility of knowledge in legal field. Patterson also emphasizes the autonomous feature of the legal system and the judicial practice as well.