Mason v. United States was a Supreme Court case decided in 1972. The case involved a challenge to the constitutionality of a federal statute that prohibited the possession of firearms by convicted felons. The petitioner, Robert Mason, was a convicted felon who had been convicted of a felony in the state of Virginia. He was subsequently charged with violating the federal statute by possessing a firearm. Mason argued that the federal statute was unconstitutional because it violated his Second Amendment right to bear arms. The Supreme Court disagreed, holding that the Second Amendment did not protect the right of convicted felons to possess firearms. The Court reasoned that the Second Amendment was intended to protect the right of citizens to bear arms in defense of themselves and their state, and that convicted felons had forfeited their right to bear arms by virtue of their criminal convictions. The Court also held that the federal statute was a valid exercise of Congress’s power to regulate interstate commerce, as the possession of firearms by convicted felons had a substantial effect on interstate commerce. The Court noted that Congress had a legitimate interest in preventing the possession of firearms by convicted felons, as such possession posed a threat to public safety. In conclusion, the Supreme Court held that the federal statute prohibiting the possession of firearms by convicted felons was constitutional. The Court reasoned that the Second Amendment did not protect the right of convicted felons to possess firearms, and that the federal statute was a valid exercise of Congress’s power to regulate interstate commerce.

In the dissenting opinion of Mason v. United States, Justice Douglas argued that the majority's decision to uphold a conviction for possession of an unregistered firearm was wrongfully decided and should be reversed. He believed that Congress had not intended to criminalize mere possession when it passed the National Firearms Act in 1934, but rather sought only to regulate commerce in firearms by requiring registration and taxation on those who wished to engage in such activities. Furthermore, he argued that since there was no evidence presented at trial showing any intent or knowledge on behalf of Mason regarding his failure to register his weapon as required by law, then he could not have been found guilty beyond a reasonable doubt under this statute. In conclusion, Justice Douglas concluded that while Congress may have wanted certain regulations with regards to firearms ownership and use within its borders, they did not intend for individuals like Mason who were simply possessing their weapons without any intention of engaging in interstate commerce or other prohibited activity would be subjecting themselves unknowingly into criminal liability through ignorance alone.