He says the plan would “end the IRS as we know it.” But even after the last stage, the federal government would still have to know people’s wages to figure out how large their Social Security checks should eventually be. And since benefits could no longer be tied to payroll taxes paid — Cain would have abolished those taxes — people would, for the first time, have an incentive to fool the federal government into thinking they had earned more than they actually did. Over-reporting income would get you higher Social Security benefits, without raising your tax liabilities. Cain wants to introduce personal accounts as part of a long-term change to Social Security, but during the decades of transition those accounts will be on top of some amount of traditional benefits. An enforcement regime of some kind would be needed.

The chief problem with the final stage of Cain’s plan, meanwhile, is that there’s no reason to believe it would work. Enforcing a 30 percent sales tax would be a nightmare, which is why no advanced economy relies on sales taxes to such an extent.

Not to worry: There’s also no reason to think the federal government would ever enact Cain’s plan. Even if, per impossibile, Cain were elected president, Congress isn’t going to tell senior citizens that, after having paid taxes on income all their lives, they will now incur extra sales taxes when they spend the money. It’s not going to raise taxes on millions of poor and middle-class people.