This section is from the book "The Constitutional Law Of The United States", by Westel Woodbury Willoughby. Also available from Amazon: Constitutional Law.
The power of taxation given to the Federal Government is comprehensive and complete, embracing all possible subjects and modes of taxation except in so far as the Constitution, in other clauses, expressly limits the power, or except in so far as limitations may be implied from the general character of the American constitutional system. The express limitations are: (1) That "all duties, imposts, and excises shall be uniform throughout the United States;"17 (2) that "no capitation or other direct tax shall be laid, unless in proportion to the census or enumeration hereinbefore directed to be taken;18 and (3) that "no tax or duty shall be laid on articles exported from any State.19
14 The clause continues: " to pay the debts and provide for the common defense and general welfare of the United States." That this is not a general grant of power to the United States to pay the debts and provide for the common defense and general welfare, but is merely a statement of the purpose for which the power to lay and collect taxes, etc., is granted. See ante, Section 22. Cf. Story, Commentaries, §§ 002-926; Tucker, Constitution, § 222; The License Tax Cases, 5 Wall. 462; 18 L. ed. 497; Knowlton v. Moore, 178 U. S. 41; 20 Sup. Ct. Rep. 747; 44 L. ed. 969.
15 For a discussion of the various definitions of excise, duty and impost, see Pacific Insurance Co. v. Soule, 7 Wall. 433; 19 L. ed. 95.
16 Hylton v. United States, 3 Dall. 171; 1 L. ed. 556.
17 Art. I, Sec. VIII, CI. 1.
The implied limitations upon the federal taxing power are those that relate to the general, if not absolute, exemption of state governmental agencies from federal interference, whether by way of taxation or otherwise,20 and those arising out of all the express limitations upon the Federal Government, which, of course, are as operative when the Federal Government is exercising its taxing powers, as it is when employing any of the other rights .possessed by it. Thus, for example, the United States may not, under the guise of a tax, take property without due process of law.
 
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