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.
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. It would declare that an act which, according to the principles and theory of our government, is entirely void, is yet, in practice, completely obligatory. It would declare that, if the Legislature shall do what is expressly forbidden, such act, notwithstanding the express prohibition, is in reality effectual. It would be giving to the Legislature a practical and real omnipotence with the same breath which professes to restrict their powers within narrow limits. It is prescribing limits, and declaring that those limits may be passed at pleasure...
The judicial power of the United States is extended to all cases arising under the Constitution.
Could it be the intention of those who gave this power to say that, in using it, the Constitution should not be looked into? That a case arising under the Constitution should be decided without examining the instrument under which it arises?
This is too extravagant to be maintained.
In some cases then, the Constitution must be looked into by the judges. And if they can open it at all, what part of it are they forbidden to read or to obey?
...Thus, the particular phraseology of the Constitution of the United States confirms and strengthens the principle, supposed to be essential to all written Constitutions, that a law repugnant to the Constitution is void, and that courts, as well as other departments, are bound by that instrument. Marbury v. Madison, 5 U.S. 137, 178-180 (1803)