Here are some of the core arguments against the legality of the income tax one finds in the tax honesty movement. Devotees probably would regard them as oversimplifications. This is certainly not an all-inclusive list.
1) The IRS declares in various documents that the income tax is "voluntary." And in Flora v. U.S. (1960), the Supreme Court announced, "Our system of taxation is based upon voluntary assessment and payment."
2) In Brushaber v. Union Pacific (1916), the Supreme Court declared that "the conclusion that the 16th Amendment provides for a hitherto unknown power of taxation" is "erroneous," and thus the 16th Amendment did not give Congress any taxing powers it did not already have. Hence, an unapportioned direct tax such as the income tax still cannot be legal. (Most mainstream readings of this extremely hard-to-follow decision say the Court meant Congress always had the power to levy an income tax, and that it was merely the question whether it should have to be apportioned that was at issue.)
3) Income, for the purposes of the tax code, should not be understood in any "common sense" way but only as defined by the Supreme Court. The Supreme Court, in Merchant's Loan and Trust Company v. Smietanka (1921), defined it as having the same meaning as in the Corporation Excise Tax of 1909-and as Irwin Schiff has written, "nothing that was received by private persons was taxable as 'income' under that Act." Income is defined as "gain derived...from labor" in a previous Supreme Court decision, Stratton's Independence v. Howbert (1913).
4) Title 26 of the U.S. Code, in which tax-related statutes are found, is inherently "void for vagueness" because it lacks precise definitions of such terms as state, United States, employee, and person. Again, "common sense" definitions aren't good enough. (Many tax honesty types interpret the use of the word includes in the tax code as properly meaning, "is limited to.")
5) According to the tax-honesty reading of U.S. Code 26, Section 861, only income from foreigners or from overseas activity appears to actually be subject to the income tax.