province and duty of the judicial department to say what the law is.... If two laws conflict with each other, the courts must decide on the operation of each.... If, then, the courts are to regard the Constitution, and the Constitution is superior to any ordinary Act of the Legislature, the Constitution and not such ordinary Act must govern the case to which they both apply. Those, then, who controvert the principle that the Constitution is to be considered in court as a paramount law, are reduced to the necessity of maintaining that courts must close their eyes on the Constitution and see only the law. This doctrine would subvert the very foundation of all written constitutions." In subsequently applying this rule, Marshall affirmed that the courts ought never to declare an Act of Congress to be void "unless upon a clear and strong conviction of its incompatibility with the Constitution." Nevertheless, the power has been constantly and frequently exercised; and there can be no doubt that from its exercise the Supreme Court of the United States derives a political importance not possessed by any other judicial tribunal. While the supremacy of the Constitution was thus judicially asserted over the acts of the national legislature, by another series of decisions its proper supremacy over acts of the authorities of the various States was in like manner vindicated. Of this series we may take as an example Cohens _v_. Virginia, decided in 1828. In this case a writ of error was obtained from the Supreme Court of the United States to a court of the State of Virginia, in order to test the validity of a statute of that State which was supposed to be in conflict with a law of the United States. It was contended on the part of Virginia that the Supreme Court could exercise no supervision over the decisions of the State tribunals, and that the clause in the Judiciary Act of 1789 which purported to confer such jurisdiction was invalid. In commenting upon this argument, Chief Justice Marshall observed that if the Constitution had provided no tribunal for the final construction of itself, or of the laws or treaties of the nation, then the Constitution and the laws and treaties might receive as many constructions as there were States. He then proceeded to demonstrate that such a power of supervision existed, maintaining that the general government, though limited as to its objects, was supreme with respect to those objects, and that such a right of supervision was essential to the maintenance of that supremacy.