Murphy v. United States was a United States Supreme Court case that addressed the issue of whether a defendant could be convicted of a crime if the evidence used to convict them was obtained through an illegal search and seizure. The case involved a man named Murphy who was convicted of receiving stolen goods after a search of his home revealed the stolen items. Murphy argued that the search was illegal and that the evidence should not be used against him. The Supreme Court ruled in favor of Murphy, holding that evidence obtained through an illegal search and seizure could not be used to convict a defendant. The Court reasoned that the Fourth Amendment of the United States Constitution protects citizens from unreasonable searches and seizures, and that the evidence obtained through an illegal search and seizure was not admissible in court. The Court also held that the exclusionary rule, which prohibits the use of illegally obtained evidence, applied to the federal government as well as to the states. This case established an important precedent in criminal law, as it established that evidence obtained through an illegal search and seizure could not be used to convict a defendant. This ruling has been cited in numerous cases since then, and has been an important part of criminal law in the United States.

Justice Field delivered the dissenting opinion in Murphy v. United States, arguing that Congress had no authority to pass a law punishing an individual for failing to appear as a witness before a grand jury. He argued that this was not within the scope of congressional power under Article I, Section 8 of the Constitution and therefore violated due process rights guaranteed by the Fifth Amendment. Furthermore, he argued that even if Congress did have such authority it would be unconstitutional because it would be an ex post facto law since it applied retroactively to offenses committed prior to its enactment. Justice Field concluded his dissent by stating that while he believed Congress should have some control over witnesses who fail or refuse to appear before grand juries, they must do so through legislation which is constitutional and does not violate any fundamental right granted by either state or federal constitutions.