Geekfire and rankandfile, the specific law that authorizes the collection of taxes is this. We have all agreed to be bound by what nine men and women in Washington tell us is the law and they have said that the government can collect these taxes. If you do not like this fact then propose a better one or even better you should reread Plato’s Crito
It describes what one should do if they disagree with the law.
Also, it does not matter what the Supreme Court decided in 1916, just like it doesn’t matter what the Supreme Court decided in 1986 when in 2003 Lawrence v. Texas overturned Bowers v Hardwick
As a matter of fact you probably have heard about people who believe the Constitution is a living document and so what it says today (or in 1916) will be different that what it says in the future (see Grutter v Bollinger) where Justice O’Conner decided that affirmative action is constitutional now but won’t be in 50 years.
So again the specific law that authorizes the IRS to collect taxes is the 16th Amendment to the Constitution which the current Supreme Court believes was correctly ratified and says they can collect the taxes.