Supervisors v. Stanley was a United States Supreme Court case that addressed the issue of whether a state could tax the income of a federal employee. The case was brought by the Supervisors of the State of Louisiana against William Stanley, a federal employee. Stanley argued that the state had no authority to tax his income, as it was derived from the federal government. The Supreme Court held that the state had the right to tax Stanley's income, as it was not derived from the federal government, but rather from his own labor. The Court reasoned that the state had the right to tax the income of its citizens, regardless of the source of the income. The Court also held that the state had the right to tax the income of federal employees, as long as the tax was not discriminatory or excessive. The Court concluded that the state had the right to tax Stanley's income, as it was not derived from the federal government.

Justice Field delivered the dissenting opinion in Supervisors v. Stanley, arguing that the tax imposed by California on foreign insurance companies was unconstitutional. He argued that Congress had exclusive power to regulate commerce with foreign nations and among states, which included regulating insurance companies from other countries doing business within a state's borders. This power could not be delegated to a state legislature or any other body of government; it was an exclusive federal right given only to Congress under Article I of the Constitution. Furthermore, he argued that if such taxes were allowed then each state would have its own set of regulations for foreign insurers operating within their boundaries, leading to confusion and disruption in interstate commerce as well as unequal treatment between domestic and foreign insurers. Justice Field concluded his dissent by stating that while taxation is necessary for governments to function properly, it must be done fairly without infringing upon rights granted exclusively by the Constitution.