We must never forget that it is a constitution we are expounding. - John Marshall

[T]he constitution controls any legislative act repugnant to it. . . . It is emphatically the province and duty of the judicial department to say what the law is. . . . 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 the very essence of judicial duty. . . . 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 be giving the legislature a practical and real omnipotence . . . The judicial power of the United States is extended to all cases arising under the constitution.

This great principle is that the Constitution and the laws made in pursuance thereof are supreme; that they control the Constitution and laws of the respective States, and cannot be controlled by them. From this, which may be almost termed an axiom, other propositions are deduced as corollaries, on the truth or error of which, and on their application to this case, the cause has been supposed to depend. These are, 1st. That a power to create implies a power to preserve; 2d. That a power to destroy, if wielded by a different hand, is hostile to, and incompatible with these powers to create and to preserve; 3d. That, where this repugnancy exists, that authority which is supreme must control, not yield to that over which it is supreme.

The powers of the legislature are defined, and limited; and that those limits may not be mistaken, or forgotten, the constitution is written. To what purpose are powers limited, and to what purpose is that limitation committed to writing, if these limits may, at any time, be passed by those intended to be restrained? The distinction, between a government with limited and unlimited powers, is abolished, if those limits do not confine the persons on whom they are imposed, and if acts prohibited and acts allowed, are of equal obligation. It is a proposition too plain to be contested, that the constitution controls any legislative act repugnant to it; or, that the legislature may alter the constitution by an ordinary act.