Mr. Cain's 9-9-9 plan was designed to be what economists call "static revenue neutral," which means that if people didn't change what they do under his plan, total tax revenues would be the same as they are under our current tax code. I believe his plan would indeed be static revenue neutral, and with the boost it would give to economic growth it would bring in even more revenue than expected.

In the recent past, federal tax revenues from the personal and business income taxes, all payroll taxes, and the capital gains, gift and estate taxes have averaged $2.3 trillion, while gross domestic product has averaged about $14.5 trillion. The total revenue from these taxes as a share of gross domestic product averages around 16%. Sometimes it's a good deal higher, as in the boom of the late 1990s, and sometimes its lower, as in today's "Great Recession." But a number in the 16%-19% range is as good as you'll get under our current tax code.

By contrast, the three tax bases for Mr. Cain's 9-9-9 plan add up to about $33 trillion. But the plan exempts from any tax people below the poverty line. Using poverty tables, this exemption reduces each tax base by roughly $2.5 trillion. Thus, Mr. Cain's 9-9-9 tax base for his business tax is $9.5 trillion, for his income tax $7.7 trillion, and for his sales tax $8.3 trillion. And there you have it! Three federal taxes at 9% that would raise roughly $2.3 trillion and replace the current income tax, corporate tax, payroll tax (employer and employee), capital gains tax and estate tax.