Harvard historian Niall Ferguson devotes part of his latest Newsweek column to a discussion of the recent focus on dividing Americans by percentile.

Last year, in the heyday of Occupy Wall Street, it was all about the 1 percent and the 99 percent. But now Democrats want to make membership of the 47 percent a badge of honor.

This language of percentiles strikes me as transitional. Americans have never been comfortable with the language of class—hence the strange phenomenon that all candidates, including both Biden and Ryan, now claim to represent the middle class. But the voters have absorbed the idea of politics as a zero-sum game, in which resources are redistributed through the systems of taxation and welfare—hence all the percents.

Yet the reality is that the real distributional issue the country faces is not between percentiles but between generations. As Paul Ryan put it in a powerful peroration, which temporarily silenced the ranting to his right, “A debt crisis is coming. We can’t keep spending and borrowing like this. We can’t keep spending money we don’t have.”

You don’t need to take this from Paul Ryan. In its latest “World Economic Outlook,” the International Monetary Fund points out that the U.S. public debt now exceeds 100 percent of GDP. The last time debt was this high, the IMF shows, the results were an “unexpected burst of inflation” and policies of “financial repression.” But that combination doesn’t look likely today—which means the debt is going to be around for years to come. More importantly, in the absence of the kind of reforms of Medicare, Social Security, and the tax system that Paul Ryan has long advocated, it’s going to keep on growing.