Writing for the Court, Justice William Brennan held that state law claims are appropriate for federal court if they form a separate but parallel ground for relief also sought in a substantial claim based on federal law. Justice Brennan explained that in cases with two distinct grounds supporting a single cause of action (but only one presents a federal question), the federal court may hear the case on the nonfederal ground. However, in cases with two distinct causes of actions (and only one is federal in character), the federal court cannot of the case on the nonfederal cause of action. Ultimately, the federal claim must have substance sufficient to confer subject matter jurisdiction on the court.

In this case, the Court was tasked with determining whether the federal court had jurisdiction over Gibb’s joint state and federal law claims. Justice Brennan explained that the §303 claims that arose under federal law were substantial and arose from the same “nucleus of operative fact” as the other claims in the case. He stated, “[w]e cannot confidently say, however, that the federal issues were so remote or played such a minor role at the trial that in effect the state claim only was tried.” Justice Brennan clarified that when a federal court has jurisdiction over a case arising under federal law, they have jurisdiction not just over the federal claim but over the case in its entirety. That includes issues of state law, so long as they come from the same set of facts. Justice Brennan concluded that the federal law claims made up a substantial part of Gibbs’ complaint, and thus federal jurisdiction was appropriate.