40. In our system of government [observes the Supreme Court], it is oftentimes difficult to fix the true boundary between the two systems, State and federal [and, adopting the words of Chief Justice Marshall, proceeds],—endeavoring to fix this boundary upon the subject of taxation, if we measure the power of taxation residing in a State by the extent of sovereignty which the people of a single State possess, and can confer on its government,—we have an intelligible standard applicable to every case to which the power may be applied. We have a principle which leaves the power of taxing the people and property unimpaired; which leaves to a State the command of all its resources, and which places beyond its reach all these powers which are conferred by the people of the United States on the government of the Union, and all these means which are given for the purpose of carrying these powers into execution. We have a principle which is safe for the States and safe for the Union.[100] We are relieved, as we ought to be, from clashing sovereignty. It follows that the powers and functions of the two governments can be harmonized “only by a wise and forbearing application of this principle.”[101] 41. A tax is a burden or charge imposed by the legislature on property or persons to raise money for public purposes.[102] The two essentials of a good tax are that it is to be laid for a public purpose and by authority. The exercise of the taxing power not only distinguishes sovereignty but also the government which sovereignty creates by delegation of power. But the State cannot exercise taxing power beyond its jurisdiction,[103] a limitation parallel to the limitation of the sovereignty of the State, that is, a version (however unphilosophical) of the idea in the phrase “residuary sovereignty.”[104] But unless restrained by the federal Constitution the power of Congress as to mode, form, or extent of taxation is unlimited. The test here is jurisdiction.[105] Taxation is the correlative of protection. As the State cannot protect so it cannot tax beyond its jurisdiction.[106] Thus the person or the property must be within the jurisdiction of the State to bring either within its taxing power. Tax laws can have no extra-territorial operation,[107] but there is no established limit of the taxing power or to the selection of objects to which it is applicable.[108] 42. A State Legislature may abuse this power, but the Constitution of the United States was not intended to furnish a corrective for every abuse of power committed by the State governments. Relief lies wholly with the electors within the State who, if the State constitution does not afford security against unjust taxation and unwise legislation, can both alter the State constitution and elect other legislators.