My new column for the FT returns to the topic of fiscal policy. When conditions allow, public borrowing needs to be brought down. Many conservatives deny that tax increases will be needed; they want public spending cuts to carry the whole adjustment, and point to Paul Ryan's Roadmap as proof that they could. To my mind, the Roadmap proves the opposite. Spending cuts won't be enough.

In a way, Mr Ryan is right: dismantling Medicare and social security is what it takes if you rely on spending cuts alone. Can it be done? No. Republicans remember what happened to the Bush plan for social security and do not intend a rerun. During the healthcare debate, they furiously opposed cuts in Medicare. Do not trust Democrats to honour these government promises, the party says. So much for Mr Ryan's plan.

You cannot hold Medicare and social security unscathed, oppose all tax increases and close the fiscal gap. The big entitlements plus defence and interest amount to some 80 per cent of federal spending. With those off the table, there is not enough left to cut.