In Moore v. United States, the Supreme Court ruled on a case involving the interpretation of federal tax law and its application to an individual who had received income from illegal activities. The defendant, Moore, was convicted for failing to report income derived from drug trafficking on his federal income tax return. He argued that requiring him to report such earnings would violate his Fifth Amendment right against self-incrimination since it would force him to disclose illegal activities. The Supreme Court disagreed with this argument and upheld Moore's conviction. They stated that while individuals have a constitutional right not to incriminate themselves in criminal matters, this protection does not extend into the realm of taxation where there is a legal obligation for all citizens - regardless of their source of income -  to file accurate returns. The court further clarified that information provided in tax returns is generally confidential and cannot be used as evidence in criminal proceedings unless certain exceptions apply.

In the dissenting opinion for Moore v. United States, Justice William Rehnquist argued that the majority's decision to suppress evidence obtained through a warrantless search of an automobile was incorrect. He contended that such searches should be allowed under certain circumstances, as established by previous court rulings. Specifically, he pointed out that in Carroll v. United States (1925), it was held permissible to conduct warrantless searches of vehicles if there is probable cause due to their mobile nature and lower expectation of privacy compared with homes or offices. Furthermore, he disagreed with the majority's interpretation of Coolidge v New Hampshire (1971) which they used as precedent for suppression; instead asserting it did not apply because this case involved immediate threat destruction or removal of evidence unlike Coolidge where police had ample opportunity to secure a warrant but didn't do so.