the so-called income tax is not a direct tax on income, but an excise tax which is an indirect tax on an event or activity. Buying or selling cigarettes, or buying and selling some cognac is an activity that requires an excise tax to be imposed on those individuals that engage in that activity.
Those activities that make one liable for a revenue tax can be found in the statutes within the Internal Revenue Code. If you are Joe the Plumber, try to find the statute that describes what a plumber does that makes him liable for the taxes imposed. And you will see that there is no such statute. The same is true of 95% of the other occupations that people are involved in.