Clapp v. Mason was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The case arose when a prisoner, Clapp, was held in a federal prison in Massachusetts and sought a writ of habeas corpus from the state court. The state court granted the writ, but the federal government argued that the state court did not have the authority to issue the writ. The Supreme Court held that the state court did not have the authority to issue the writ of habeas corpus. The Court reasoned that the writ of habeas corpus was a federal power, and that the state court did not have the authority to interfere with the federal government's power to imprison individuals. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to imprison individuals. The Court's decision in Clapp v. Mason established that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The decision also established that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to imprison individuals.

Justice Field delivered the dissenting opinion in Clapp v. Mason, arguing that the majority's decision was contrary to established precedent and would lead to an unjust result. He argued that under prior decisions of the Supreme Court, a contract between two parties could not be altered by one party without consent from both sides. In this case, he noted that there had been no agreement or understanding between Clapp and Mason regarding any alteration of their original contract; thus, it should remain binding on both parties as originally written. Furthermore, Justice Field argued that if either party were allowed to unilaterally alter a contract after its formation without mutual assent from all involved parties then contracts would become meaningless documents with little legal effect since they could easily be changed at will by any one side whenever desired. As such, he concluded that allowing for unilateral alterations of contracts would create chaos in commercial transactions and ultimately lead to injustice rather than justice being served as intended by law.