Cowles v. Mercer County was a United States Supreme Court case that dealt with the issue of taxation. The case involved a dispute between the plaintiff, Cowles, and the defendant, Mercer County, over the taxation of a piece of property owned by Cowles. Cowles argued that the tax imposed by Mercer County was unconstitutional because it was based on an assessment of the property that was higher than its actual value. The Supreme Court agreed with Cowles, ruling that the tax was unconstitutional because it was based on an assessment that was higher than the actual value of the property. The Court held that the tax was a violation of the Fourteenth Amendment, which guarantees equal protection under the law. The Court also held that the tax was a violation of the due process clause of the Fourteenth Amendment, which requires that taxes be imposed in a fair and reasonable manner. The Court's ruling in this case established the principle that taxes must be based on the actual value of the property being taxed.

In Cowles v. Mercer County, the Supreme Court was asked to decide whether a state court had jurisdiction over an action brought by a non-resident plaintiff against a resident defendant in which the cause of action arose outside of the state. The majority opinion held that such jurisdiction did exist and affirmed the judgment of the lower court. In dissent, Justice Field argued that allowing states to exercise this type of extraterritorial jurisdiction would lead to confusion and conflict between different states' laws on similar matters, as well as create unequal treatment for citizens depending on their residence status. He further noted that it could be seen as an infringement upon Congress's power to regulate interstate commerce under Article I, Section 8 of the Constitution. Ultimately he concluded that since no federal law authorized such jurisdictional authority for individual states, they should not have been allowed to assume it themselves without congressional approval or constitutional amendment.