Cousin v. Generes 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 Generes, was held in a federal prison in Louisiana. Generes' cousin, John Cousin, filed a petition for a writ of habeas corpus in a Louisiana state court, seeking to have Generes released from federal custody. The federal government argued that the state court did not have the authority to issue the writ, as the federal government had exclusive jurisdiction over federal prisoners. The Supreme Court ultimately 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 remedy, and that the state court did not have the power to interfere with the federal government's exclusive jurisdiction over federal prisoners. The Court also noted that the writ of habeas corpus was a remedy of last resort, and that the state court should not interfere with the federal government's exclusive jurisdiction over federal prisoners. 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 remedy, and that the state court did not have the power to interfere with the federal government's exclusive jurisdiction over federal prisoners.

In the case of Cousin v. Genereux, the Supreme Court was tasked with determining whether a Louisiana law that allowed for slaves to be emancipated upon their master's death was constitutional. The majority opinion held that it was not, as Congress had exclusive power over slavery and emancipation in territories acquired from France under the Louisiana Purchase Treaty. Justice Field dissented from this ruling on two grounds: firstly, he argued that since no federal statute prohibited such laws at the time of its enactment by Louisiana, it should stand; secondly, he maintained that even if Congress did have exclusive power over slavery and emancipation in these territories, they could not retroactively invalidate state laws already enacted prior to any congressional action taken on those matters. In conclusion then, Justice Field believed that states were free to pass legislation regarding slavery within their own borders unless expressly forbidden by Congress or otherwise unconstitutional.