Spats, what you quoted from
Marbury v. Madison is often quoted out of context.
The whole paragraph you excerpted the follwing from: ...
...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....
... in its entirety is as follows, with the previous paragraph added to keep it in context:
If an act of the legislature, repugnant to the constitution, is void, does it, notwithstanding its invalidity, bind the courts and oblige them to give it effect? Or, in other words, though it be not law, does it constitute a rule as operative as if it was a law? This would be to overthrow in fact what was established in theory; and would seem, at first view, an absurdity too gross to be insisted on. It shall, however, receive a more attentive consideration.
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. [5 U.S. 137, 178] So if a law be in opposition to the constitution: if both the law and the constitution apply to a particular case, so that the court must either decide that case conformably to the law, disregarding the constitution; or conformably to the constitution, disregarding the law: the court must determine which of these conflicting rules governs the case. This is of the very essence of judicial duty.
(* Interpret that "rule," not the Constitution or a law.)
Basically, the Court said it must determine which law has supremacy, the Constitution or the legislation. The court said it must "interpret that rule," meaning the Court must decide to follow either the rule that the Constitution is supreme or a supposed rule that the law should be adjudicated regardless of its constitutionality. The "rule" that is the subject of the paragraph you excerpted a portion from is the "rule" mentioned in the previous paragraph. You are applying a non sequitur if your intent to post that line from
Marbury v. Madison was to imply that the Court said it must "interpret" the Constitution.
Obviously, all the Court needed to do was abide the supremacy clause in Article VI.
Woody