Simpson v. Greeley 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, John Simpson, was held in a federal prison in the state of New York. Simpson sought a writ of habeas corpus from the state court, claiming that he was being held in violation of his constitutional rights. The state court granted the writ, and the federal government appealed the decision to the Supreme Court. The Supreme Court held 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 Court reasoned that the writ of habeas corpus was a federal prerogative, 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 protect the rights of individuals. In conclusion, the Supreme Court held 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 Court reasoned that the writ of habeas corpus was a federal prerogative, 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 protect the rights of individuals.

Justice Field delivered the dissenting opinion in Simpson v. Greeley, arguing that the majority's decision was contrary to established precedent and would lead to a dangerous expansion of federal power. He argued that Congress had no authority under the Constitution to pass laws regulating private contracts between citizens of different states, as it did with the Act of 1866 at issue in this case. Furthermore, he noted that if such an expansive interpretation were accepted then Congress could effectively regulate all aspects of life within each state by passing similar legislation on any number of topics. This would be a violation not only of state sovereignty but also individual liberty since it would allow for arbitrary interference from Washington into matters traditionally left up to local governments or individuals themselves. Justice Field concluded his dissent by noting that while he sympathized with Mr. Simpson's plight, there was simply no legal basis for overturning existing law and allowing him relief through federal action rather than seeking redress through other means available under state law or contract theory