The 16th Amendment:

After passage of the 16th Amendment, Congress immediately wrote a new income tax law, which was quickly challenged (as with all previous challenges, by someone with federally-connected income). In its decision the Supreme Court, in speaking of the confusion created, said:

"We are of opinion, however, 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..." (Brushaber v. Union Pacific Railroad Co. 240 US 1) (1916)

This ruling has been referenced more than 50 times in later rulings. Later the court also wrote:

"The Sixteenth Amendment, although referred to in argument, has no real bearing and may be put out of view. As pointed out in recent decisions, it does not extend the taxing power to new or excepted subjects, but merely removes all occasion, which otherwise might exist, for an apportionment among the states of taxes laid on income, whether it be derived from one source or another." (Peck v. Lowe, 247 U.S. 165) (1918)

The court was reinforcing what had been the case all along: the federal government always had the right to indirectly tax 'income', understood properly as earnings derived from federal privilege. The 16th Amendment gave Congress no new powers of taxation, it merely removed from future Court decisions any consideration of the 'source' of that income. In this sense, the 16th Amendment is NOT the basis of the current income tax, much as the government might like you to believe it is.

As an aside, we note that in several places on its website the IRS declares that the income tax is a direct tax based on the 16th Amendment. For example, in 2009 the Taxpayer Advocate Service, a part of the IRS, said, "Income taxes are direct taxes, but do not have to be apportioned because of the Sixteenth Amendment" here (PDF, p.67, footnote 155). Why they tell such a blatant lie should become apparent as we proceed.