After the last debate, I noted that Ben Carson’s tax math didn’t add up. Here’s what he said about a 15 percent flat tax: “[I]f 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.” I went through some of the reasons this would not work. A post by James Downie raises an additional reason: Carson’s assuming the federal government can raise money by taxing itself (and state governments too). Federal spending is included in that $18 trillion, and accounts for half a trillion of the revenue he was counting on. The Carson campaign has said that it will put out a tax plan in the next few weeks; presumably it will reflect more thought than his debate remarks.