Dworkin argues that sometimes, judges and lawyers argue about “what the law is.”
My take on this is that in a complex society such as ours, there are numerous lawgivers and sources of law, as well as types of law. For example, there is natural law vs. positive law, tort / contract / criminal law, family law, nuisance law, law and economics, federal / state / local law, constitutional law, case law, government regulations, and others.
It may happen that some of these laws contradict each other. It may also happen that a certain lawgiver is not recognized as such in a case; his authority for whatever reason is questioned. Finally, judges themselves make or discover law.
As a result, (1) judges may disagree with each other as to what constitutes proper natural law; and (2) each lawgiver is supposed to integrate any law he promulgates into the totality of the legal system such as to keep it coherent, and this process is rife with difficulties.
These considerations entail that “What is law?” may be a non-trivial question for a judge deciding a particular case.