Josh Barro, in the NYT, examines the GOP candidates’ tax plans. For Ben Carson’s 10% “tithe plan”, Barro notes:

Mr. Carson disclaimed that idea. “Well, first of all, I didn’t say that the rate would be 10 percent,” he told Ms. Quick. Well, actually, he did. At the first Republican debate, he described the plan as follows: “You make $10 billion, you pay a billion. You make $10, you pay one.” But anyway, Mr. Carson now doesn’t think a 10 percent rate is workable, and said the rate would have to be closer to 15 percent.

He argued: “If you’re talking about an $18 trillion economy, you’re talking about a 15 percent tax on your gross domestic product. You’re talking about $2.7 trillion. We have a budget closer to $3.5 trillion. But if you also apply that same 15 percent to several other things, including corporate taxes, and including the capital gains taxes, you make that amount up pretty quickly.”

Essentially, he’s saying, if the base is broad enough, a 15 percent tax will suffice. The main problem is his assumption that you can have a tax with a base equal to 100 percent of the economy. Some things are always tax exempt, for a mixture of good reasons and bad. We don’t tax investment income earned by nonprofits like churches and universities, nor do we tax investment returns to pension funds and 401(k) retirement accounts. We don’t tax income that people give away to charity. We don’t tax employee health benefits. Most tax plans decline to tax at least some income for low earners….

Yeah, Carson apparently thinks we take the flat tax thing and apply it to the entire economy, to the whole GDP thing.