Marshall v. Knox was a United States Supreme Court case that dealt with the issue of whether a state could tax the income of a non-resident. The case was brought by a resident of the state of New York, who was being taxed on income he had earned in the state of Pennsylvania. The plaintiff argued that the taxation of his income was unconstitutional, as it violated the privileges and immunities clause of the Fourteenth Amendment. The Supreme Court ultimately ruled in favor of the plaintiff, finding that the taxation of the plaintiff's income was unconstitutional. The Court held that the taxation of the plaintiff's income was a violation of the privileges and immunities clause of the Fourteenth Amendment, as it discriminated against non-residents. The Court reasoned that the taxation of the plaintiff's income was a form of discrimination against non-residents, as it placed a greater burden on them than on residents of the state. The decision in Marshall v. Knox was an important one, as it established the principle that states cannot discriminate against non-residents when it comes to taxation. This decision has been cited in numerous cases since, and has been used to protect the rights of non-residents in other contexts as well.

In Marshall v. Knox, the Supreme Court was asked to decide whether a state court had jurisdiction over an action brought by a citizen of one state against another in which the defendant resided in yet another state. The majority opinion held that such suits could not be maintained without violating the Constitution's Full Faith and Credit Clause, as it would require states to recognize judgments from other states with which they have no connection or interest. Justice Field dissented on this point, arguing that Congress has exclusive power under Article IV of the Constitution to prescribe rules for determining when judgments rendered by courts of one State shall be respected and enforced in other States; thus, he argued that individual citizens should not be denied access to justice simply because their suit involves parties from different states. He further noted that there is nothing unconstitutional about allowing individuals who are residents of different states to bring actions against each other before any court having competent jurisdiction over them both; rather than denying these rights altogether due solely to geographical differences between litigants, he suggested instead allowing full faith and credit only if certain conditions were met (e.g., proper service).