SEMPLE v. HAGAR was a United States Supreme Court case that was decided in 1866. The case involved a dispute between two parties over the ownership of a slave named Hagar. The plaintiff, Semple, claimed that he had purchased Hagar from a third party and that the defendant, Hagar, had wrongfully taken possession of her. The Supreme Court held that the plaintiff had failed to prove his ownership of the slave and that the defendant was entitled to her possession. The Court reasoned that the plaintiff had failed to provide sufficient evidence to prove his ownership of the slave. The Court noted that the plaintiff had not provided any written evidence of the purchase, nor had he provided any witnesses to testify to the transaction. Furthermore, the Court noted that the plaintiff had failed to provide any evidence of the slave's identity or of her prior ownership. As a result, the Court held that the defendant was entitled to her possession. In its decision, the Court also noted that the plaintiff had failed to provide any evidence of the slave's value or of the consideration paid for her. The Court held that the plaintiff had failed to prove his ownership of the slave and that the defendant was entitled to her possession. This decision established the principle that a plaintiff must provide sufficient evidence to prove his ownership of a slave in order to be successful in a dispute over ownership.

In Semple v. Hagar, the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving an Indian tribe and non-Indians. The majority opinion held that the state court did not have jurisdiction because it would interfere with tribal sovereignty and violate federal law. However, Justice Field dissented from this decision on two grounds: first, he argued that Congress had never explicitly prohibited states from exercising their authority in such cases; second, he argued that if Congress intended to limit state power in these matters then they should have done so more clearly than they had done thus far. He concluded by stating his belief that allowing states to exercise their powers in such cases would be beneficial for both Indians and non-Indians alike as it could help resolve disputes without resorting to violence or other extreme measures.