There are several common conversational gambits among tax protestors, but the most common is: “What law, specifically, requires us to pay income taxes?†That's as good an introduction as any to the subject. So we can deal with it quickly: the law is Title 26 of the United States code, Subtitle A, Chapter 1, Subchapter A, part I. It starts with “There is hereby imposed on the taxable income … a tax determined in accordance with the following table.â€
The Code goes into an excruciating amount of detail about what is taxable, what is not, who must pay, what doesn't count, etc. etc. etc. Many tax protestors will read this over and argue that even though it says what the taxes are, how they're calculated, and the like, it never actually says you have to pay them.
But just try that argument with something outside the tax code. If you're given a fifteen-page description of the duties of a professional mold removing gell salesman, and are offered and accept the salary of a mold removing gell salesman, but then argue that all these duties never specified that you had to perform them (but you'll cash the checks, thank-you-very-much) you wouldn't get too far.
Tax protestors are like the mythical mold removing gell salesman. Even if the tax code is not written so that, for every possible reading, the intended payers are the actual payers, the courts have ruled that, essentially, the document means what its writers meant it to, and grammar be damned you have to pay what the code says you owe.