The Sixteenth Amendment of 1913 abolishes the limitation of apportionment or enumeration in the imposition and collection of an income tax. The Income Tax law of October 3, 1913—the first of the kind enacted by Congress under the amendment—exempted incomes of $3000, or less, or $4000, or less, as the person taxed may be single or married. The amount of the exemption is fixed at the discretion of Congress. So too is the rate of taxation by duties, imposts, and excises, as well as the inclusion or exclusion of articles subject to them, but Congress must make such taxes uniform throughout the United States.[125] The taxing power may be used to encourage or to discourage an activity, or to destroy it. As thus used, the exercise of the taxing power, whether by the State or by the United States, may characterize the policy, or administration of its government. So too if a State engages in manufacturing, or in any activity or occupation taxable under federal revenue laws, it is amenable in taxes like a private person.[126] CHAPTER V THE LAW OF COMMERCE 49. The power to regulate commerce belongs to sovereignty. By the Constitution Congress is empowered “to regulate commerce with foreign nations, and among the several States, and with the Indian tribes.”[127] The principle of this regulation, or of the exercise of the power, is essentially that of taxation: it is a matter of jurisdiction. “The power of Congress to regulate commerce,” observes Chief Justice Marshall, in the first American judicial decision on the subject, “comprehends and warrants every act of national sovereignty which any other sovereign nation may exercise.”[128] The enormous powers Congress wields through this clause cannot be fully defined. The Supreme Court has not defined them. Like sovereignty itself, the exercise of its essential powers, even when delegated functionally in government, does not yield to the limits of definition. The decisions of the Supreme Court are not definitions of the power over commerce so much as they are definitions of the particular exercise of the power of Congress within its jurisdiction, with respect to commerce, by the Constitution.[129] For the States also have jurisdiction over commerce. Our knowledge of the boundaries of these two jurisdictions arises from the conflict of laws concerning them. 50. In defining national jurisdiction and State jurisdiction over commerce, two propositions are fundamental: (1) The Constitution of the United States is the supreme law of the land.[130] (2) It is the province and duty of the judicial department to say what the law is.[131]