It's interesting how SCOTUS, deliberately and narrowly defined the nebulous language of 16th Amendment in the first cases which sought to test its bounds.
Generally speaking there are only two types of taxes allowable under the Constitution: direct and indirect. A direct tax, is a tax which
cannot be avoided by normal activity; it is a tax that can be imposed on the enjoyment of Rights, of property/income, and which, constitutionally,
must be apportioned. An indirect tax, is a tax that
can be avoided by normal activity; it is an excise tax, a tax on specific activities: the exercise of privileges.
There's no room here to delve into the reasoning behind the need for the 16th Amendment, expect to say that Congress was eager to capture
corporate income (increase from capital gains). Needless to say, Congress gave it several addled attempts, and the court shot them down, one after the other.
So, with that in mind, it's interesting to note the court's decisions respecting the breadth of the 16th Amendment. To recap, without apportionment i.e. a direct tax, an individual
cannot be taxed for the privilege of existing. Or, looking at it from the other side, individuals are not subject to an indirect tax (an excise), unless they engage in a privileged (corporate) activity.
From
Brushaber, the first case, Justice White states (and note, he's using a negative approach to clarify the extent of the Amendment by drawing attention to "
erroneous assumption", a position which he later hammers crystal clear in Stanton):
The various propositions are so intermingled as to cause it to be difficult to classify them. We are of opinion, however, [240 U.S. 1, 11] that the confusion is not inherent, but rather arises from the conclusion that the 16th Amendment provides for a hitherto unknown power of taxation; that is, a power to levy an income tax which, although direct, should not be subject to the regulation of apportionment applicable to all other direct taxes. And the far-reaching effect of this erroneous assumption will be made clear by generalizing the many contentions advanced in argument to support it, as follows: (a) The Amendment authorizes only a particular character of direct tax without apportionment, and therefore if a tax is levied under its assumed authority which does not partake of the characteristics exacted by the Amendment, it is outside of the Amendment, and is void as a direct tax in the general constitutional sense because not apportioned. . .
Did you catch that? The suggestion that the 16th Amendment provided an unknown power of taxation or a power to levy an income tax not subject to apportionment, is an erroneous assumption.
Now, here again, is White opining in Stanton:
. . . by the previous ruling it was settled that the provisions of the 16th Amendment conferred no new power of taxation, but simply prohibited the previous complete and plenary power of income taxation possessed by Congress from the beginning from being taken out of the category of indirect taxation to which it inherently belonged. . .
What do we know? The 16th Amendment, per SCOTUS, did not disrupt apportionment or uniformity, it did not upset "income taxation" per the Constitution into another category, nor did it grant Congress new plenary powers of taxation respecting the work-a-day individual's property i.e. his/her income. What it did do however, was give Congress the power to tax gain or increase from corporate activities.
I think it should be noted that the income tax is not a tax on income per the 1909 Corporate Tax Act and the 16th Amendment; but rather, it is an excise tax with respect to engaging in an activity or privilege, and it uses the amount of income as a gauge to the level of taxation.
So, if you're not engaged in a privileged activity, and income taxation is yet in the category of direct taxation, how can the federal gov't, without apportionment, infringe on the enjoyment of your Rights and think to gnaw away at your property/income?
Good freakin' question.