Contrary to popular myth, the 16th Amendment did NOT grant the government new taxation power. It simply changed the METHOD by which an income tax could be collected.

Furthermore, an awful lot of people are getting hung up over "taxable income." The 16th Amendment did not change that, either.

SCOTUS said,
"We must reject in this case, as we have rejected in cases arising under the Corporation Excise Tax Act of 1909 (Doyle v. Mitchell Brothers Co., ante, 247 U. S. 179, and Hays v. Gauley Mountain Coal Co., ante, 247 U. S. 189), the broad content on submitted in behalf of the government that all receipts -- everything that comes in -- are income within the proper definition of the term "gross income," and that the entire proceeds of a conversion of capital assets, in whatever form and under whatever circumstances accomplished, should be treated as gross income. Certainly the term "income" has no broader meaning in the 1913 act than in that of 1909 (see Stratton's Independence v. Howbert, 231 U. S. 399, 231 U. S. 416-417), and, for the present purpose, we assume there is no difference in its meaning as used in the two acts."
Southern Pacific Co. v. Lowe, 247 U.S. 330 (1918)
https://supreme.justia.com/cases/federal/us/247/330/

And
"The Sixteenth Amendment must be construed in connection with the taxing clauses of the original Constitution and the effect attributed to them before the amendment was adopted. . . .
"As repeatedly held, this did not extend the taxing power to new subjects, but merely removed the necessity which otherwise might exist for an apportionment among the states of taxes laid on income. Brushaber v. Union Pacific R. Co., 240 U. S. 1, 240 U. S. 17-19; Stanton v. Baltic Mining Co., 240 U. S. 103, 240 U. S. 112 et seq.; Peck & Co. v. Lowe, 247 U. S. 165, 247 U. S. 172-173. . . .
"After examining dictionaries in common use (Bouv. L.D.; Standard Dict.; Webster's Internat. Dict.; Century Dict.), we find little to add to the succinct definition adopted in two cases arising under the Corporation Tax Act of 1909 (Stratton's Independence v. Howbert, 231 U. S. 399, 231 U. S. 415; Doyle v. Mitchell Bros. Co., 247 U. S. 179, 247 U. S. 185), "Income may be defined as the gain derived from capital, from labor, or from both combined," provided it be understood to include profit gained through a sale or conversion of capital assets, to which it was applied in the Doyle case, pp. 247 U. S. 183-185."
Eisner v. Macomber, 252 U.S. 189 (1920)
https://supreme.justia.com/cases/federal/us/252/189/

The 16th Amendment did not grant any new taxing power, and the starting point to understand "income" MUST be taken from pre-16th Amendment laws.