Deductive legal reasoning fails where no single controlling rule exists.

Evidence

Why is the deductive model not a fully adequate description of judicial decision making? For legal reasoning to follow the deductive model, there must be a single legal rule for the judge to apply in deciding the case. But there may be cases for which there is no controlling precedent or established legal rule—for instance, when a judge interprets a recently enacted statute, addresses a novel constitutional claim, or confronts an unprecedented situation at common law. For other cases, there may be more than one category or rule that seems applicable. In fact, the issue in dispute may be which rule to apply. Thus, the deductive model of judicial decision making cannot explain how judges deal with novel issues or choose between competing rules. — G. Alan Tarr, Judicial Process and Judicial…, 2520

Categorization

• DEFINE: States the necessary precondition of the deductive (formalist) model: one applicable legal rule, which the passage then shows is often absent.
• XREF: Connects to the legal-formalism-versus-realism debate and to course themes on the sources judges draw on when precedent is silent.