Aside from the general limitations resulting from the requirements of due process of law and equal protection of the laws, the federal constitution contains some specific limitations on the state taxing power. The laying and collection of duties, imposts, and excises is a legitimate method of exercising the power of taxation; but by the federal constitution authority to raise taxes by this method is specifically conferred on the federal government (Art. I, § 8, ¶ 1), and while this does not in itself exclude the exercise of like power by the states, nevertheless it is specifically provided that "No state shall without the consent of the Congress lay any imposts or duties on imports or exports, except what may be absolutely necessary for executing its inspection laws" (Art. I, § 10, ¶ 2), and that "No state shall without the consent of Congress lay any duty of tonnage" (Art. I, § 10, ¶ 3). These restrictions, while they relate specifically to the state power to tax, seem to be intended to prevent interference by the states with freedom of commercial intercourse. The provision that Congress shall have power " To regulate commerce with foreign nations and among the several states and with the Indian tribes" (Art. I, § 8, ¶ 3), has been interpreted also as restricting the levying of state taxes on foreign or interstate commerce. The extent to which this clause restricts state taxation will be considered in the chapter relating to the regulation of commerce. (See below, § 92.) The validity of state inspection laws, and of state taxes on boats and vessels, has been the subject of some discussion, but the matter is not of sufficient general importance to justify further elaboration. It is sufficient to say that few attempts have been made by the states to impose taxes for the enforcement of state inspection; and that taxes on boats and vessels have been sustained where they are reasonably calculated to reach the value of the property itself, so far as it is within the jurisdiction of the states, the tonnage tax prohibited being considered to be a tax on boats and vessels based upon their capacity rather than their value.