Writing his opinion, Chief Justice John Marshall observed that Congress must not be allowed to pass legislation that was contrary to the Constitution. Marshall said that in authorizing original jurisdiction for the Court in some cases in the Judiciary Act of 1789, Congress had surpassed the intent of Article III of the Constitution. It was obvious, Marshall said, that this was impermissable:
...the constitution controls any legislative act repugnant to it; or, that the legislature may alter the constitution by an ordinary act Between these alternatives there is no middle ground. The constitution is either a superior, paramount law, unchangeable by ordinary means, or it is on a level with ordinary legislative acts, and like other acts, is alterable when the legislature shall please to alter it.
He went on to say that the Constitution could never have meant for the courts to enforce laws that were, essentially, unconstitutional. This would mean that the Constitution was completely nonbinding. Moreover, he asserted the right of the courts to void any unconstitutional legislation by explaining the role of the judiciary as he understood it:
It is emphatically the province and duty of the judicial department to say what the law is. Those who apply the rule to particular cases must of necessity expound and interpret that rule. If two laws conflict with each other, the courts must decide on the operation of each.
So the very nature of the judiciary required it to interpret law and the Constitution and apply it to specific circumstances. Thus the power to determine the constitutionality of laws would most logically be vested in the courts. The irony of the case is often overlooked: The Judiciary Act actually granted the federal courts considerable powers. By ruling that one of the clauses in the Act was unconstitutional, Marshall limited the Court's jurisdiction. But in so doing, he asserted the principle of judicial review, which strengthened the Court immeasurably.