Cooper v. Reynolds was a United States Supreme Court case that was decided in 1870. The case involved a dispute between two parties over a contract for the sale of a steamboat. The plaintiff, Cooper, had contracted with the defendant, Reynolds, to purchase a steamboat for $2,000. Cooper paid the full amount, but Reynolds refused to deliver the boat. Cooper then sued Reynolds for breach of contract. The Supreme Court held that Reynolds was liable for breach of contract. The Court found that the contract was valid and enforceable, and that Reynolds had breached it by failing to deliver the boat. The Court also held that Cooper was entitled to damages for the breach, and that he was entitled to recover the full amount of the purchase price, plus interest. The Court's decision in Cooper v. Reynolds established the principle that a party who breaches a contract is liable for damages, and that the damages should be equal to the amount of the contract. This decision has been cited in numerous subsequent cases, and is still an important part of contract law today.

In Cooper v. Reynolds, the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving two citizens of different states. The majority opinion held that it did not have such jurisdiction and dismissed the appeal. In his dissenting opinion, Justice Field argued that under Article III of the Constitution, Congress has given federal courts exclusive original jurisdiction in cases where one party is an alien or citizen of another state. He further argued that this power should be extended to state courts as well since they are part of the judicial system established by Congress and thus should be allowed to exercise their own authority in matters concerning diversity between parties from different states. Furthermore, he noted that allowing state courts to hear these types of cases would help ensure uniformity among decisions made across all jurisdictions which could ultimately lead to greater harmony among them all.