Downes and Company vs. Edward B. Church was a case heard by the United States Supreme Court in 1839 that dealt with the issue of whether or not goods imported from abroad could be taxed without violating the Constitution’s ban on ex post facto laws. The court ruled that such taxes were unconstitutional, as they would retroactively punish those who had already purchased foreign goods before their enactment. This decision established an important precedent for future cases involving taxation of imports, as it set forth a clear standard for determining when such taxes are permissible under the law. Furthermore, this ruling also served to reinforce constitutional protections against arbitrary government action and ensured that citizens would not be punished after-the-fact for engaging in lawful activities prior to any new legislation being passed into law.

In Downes and Company vs. Edward B. Church, the Supreme Court was asked to decide whether a New York state law that imposed taxes on goods imported from foreign countries violated the Constitution's Import-Export Clause. The majority opinion held that it did not violate the clause because Congress had not specifically prohibited such taxation by states. However, Justice McLean dissented from this decision and argued that allowing states to impose taxes on imports would be unconstitutional as it would interfere with Congress' exclusive power over international commerce granted by Article I of the Constitution. He further argued that if states were allowed to tax imports then they could also regulate them in other ways which would undermine congressional authority over trade between nations and among different parts of the United States. In conclusion, Justice McLean believed that allowing states to tax imports was an unconstitutional infringement upon Congressional power under Article I of the Constitution and should therefore be struck down as invalid