To understand the reasoning of an opinion, you should first identify the source of the law the judge applied. Some opinions interpret the Constitution while others interpret ‘statutes’, still other cases interpret ‘the common law’, which is a term that usually refers to the body of prior case decisions.
The source of law is very important because common law follows a clear hierarchy. Constitutional rules trump statutory rules, and statutory rules trump common law rules.
After you have identified the source of law, you should next identify the method of reasoning that the court used to justify its decision. When a case is governed by a statute, for example, the court usually will simply follow what the statute says. The court’s role is narrow in such settings because the legislature has settled the law. Similarly, when past courts have already answered similar questions before, a court may conclude that it is required to reach a particular result because it is bound by the past precedents. This is an application of the judicial practice of ‘stare decisis’, an abbreviation of a Latin phrase meaning ‘That which has been already decided should remain settled’.
In other settings, courts may justify their decisions on public policy grounds. That is, they may pick the rule that they think is the best rule, and they may explain in the opinion why they think that rule is best. This is particularly likely in common law cases where judges are not bound by a statute or constitutional rule. Other courts will rely on morality, fairness, or notions of justice to justify their decisions. Many courts will mix and match, relying on several or even all of these justifications. (Kerr, 2007)
