Remarks on the Meaning of "Income"
Amendment 16 says only: "The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration" and so fails to define the term "income". A whole mess of confusion has followed - and even preceded its apparent 1913 ratification.
Its other major terms are defined elsewhere in the Constitution; for example "congress" is the subject of the whole of Article 1 and "apportionment" is explained in Article 1 Section 9. But "income" itself, by far the most important term used, is not. Given that Amendment 16 is said to be the clause enabling the whole of the trillion dollar a year "income tax", that omission may be highly significant.
Read more here.
Usage of the term "income" is very far from uniform and unambiguous, so it should not be used in law without a definition and only a new Amendment can legally produce one. Since in Subtitle A of Title 26 it has been so used anyway, that Subtitle - the entire alleged "income tax" - is based upon a literally meaningless term and is, therefore, void.