The subject is the execution of those great powers on which the welfare of a Nation essentially depends. It must have been the intention of those who gave these powers to insure, so far as human prudence could insure, their beneficial execution. This could not be done by confiding the choice of means to such narrow limits as not to leave it in the power of Congress to adopt any which might be appropriate, and which were conducive to the end. This provision is made in a Constitution intended to endure for ages to come, and consequently to be adapted to the various crises of human affairs. To have prescribed the means by which Government should, in all future time, execute its powers would have been to change entirely the character of the instrument and give it the properties of a legal code. It would have been an unwise attempt to provide by immutable rules for exigencies which, if foreseen at all, must have been seen dimly, and which can be best provided for as they occur. To have declared that the best means shall not be used, but those alone without which the power given would be nugatory, would have been to deprive the legislature of the capacity to avail itself of experience, to exercise its reason, and to accommodate its legislation to circumstances. [. . .]

[. . . W]e think the sound construction of the Constitution must allow to the national legislature that discretion with respect to the means by which the powers it confers are to be carried into execution which will enable that body to perform the high duties assigned to it in the manner most beneficial to the people. Let the end be legitimate, let it be within the scope of the Constitution, and all means which are appropriate, which are plainly adapted to that end, which are not prohibited, but consistent with the letter and spirit of the Constitution, are Constitutional.

(Emphasis supplied.) But what did John Marshall know anyway? Now Scalia, he could tell you.